Where do the characters get the stuff they use In the Mario Kart Games from? And did the characters design the tracks that have their names?
I’m not quite sure what stuff you’re specifically referring to, but regardless, the answer is quite complicated. Assuming you’re talking about Karts and Items, the answer is “various places”.
Back in Super Mario Kart, while Mario was likely the one who organised, funded or at least initially proposed the event, Lakitu was the coordinator of the event, and ran the tracks. In the beginning they had more conservative rules surrounding the vehicles in use on the courses:
Hi everybody! Thanks for dropping by the Super Mario Kart race track. My name is Lakitu, I run this track. If you need my help, I'll be around. We race one seater, metal pipe frame go carts here. There are two engine sizes to choose from: 50cc or 100cc.
[Source: INTRODUCTION, Page 2, Instruction Booklet of Super Mario Kart, September 1992]
Mario Kart stated out a a fun recreational event with a handful of trophies. That’s the main reason the characters take part, and it has been the reason since the very beginning, in Super Mario Kart. Each one of the characters is competing due to their connection to Mario.
An adorable little kart racetrack has been built in the Mario world! Mario and his happy-go-lucky friends have gotten so enthusiastic about kart racing that they're already causing a ruckus! Even now, they're smack in the middle of a race! Let's take a quick peek at how it's going, shall we? Oh, it's just about to start! Even the usually cute characters look a little tense. Let's watch Mario and friends fight hard!
[Source: Intro, Japanese Store Promo for Super Mario Kart, August 1992]
However, in the years since it has become the Mushroom Kingdom’s premiere racing event and grown significantly. Racers from various Kingdoms take part resulting in an incredibly diverse audience. Mario Kart now has dozens of sponsors for each Grand Prix, extended tournaments, its own television network, branded tracks and inter-dimensional courses and guest racers.
Mario Kart 64 was the first major step in this direction, but Mario Kart: Double Dash!! brought things even further. From its manual:
THE MUSHROOM KINGDOM’S PREMIER RACING EVENT IS BACK! HOWEVER, THERE’S BEEN ONE MAJOR CHANGE TO THE RULES OF THE ROAD… THERE ARE NOW TWO RIDERS PER KART! NOW, RIDERS FROM THROUGHOUT THE LAND HAVE COME TOGETHER NO TEST THEIR DRIVING SKILLS. WHICH PAIR WILL PROVE THE FASTEST?
[Source: The Double Dash Grand Prix!, Page 6, Instruction Booklet of Mario Kart: Double Dash!!, November 2003]
When Mario Kart started out, standard Pipe Frames were the only legal builds for the Mario GP. These models were likely built by Mario and his friends themselves.
However, with Mario Kart: Double Dash!! the Pipe Frames were retired in favour of a variety of Karts. Many of these karts were custom-made or modified pre-existing cars. Take Luigi's kart of choice in Mario Kart DS, for example: the Poltergust 4000. This Kart is part of the Poltergust series, a line of inventions created by Professor E. Gadd.
Another pair of examples are the Wario Car, which Wario makes use of in Mario Kart: Double Dash!! and Mario Kart Arcade GP 2, and the Wario Bike, which Wario makes use of in Mario Kart Wii. These are Wario's personal car and bike, which he uses in the Wario Land and WarioWare titles, and both were built by Wario's friend Dr. Crygor:
I AM DR. CRYGOR, THE GENIUS SCHOLAR. OTHERS HAVE SAID A "DANGEROUS" SCHOLAR, BUT I PAY NO MIND TO THEIR OPINIONS. MY LABORATORY IS SET UP TO FURTHER MY EXPERIMENTS IN REALITY GAMES. FEEL FREE TO STOP BY AND CONTRIBUTE TO MY RESEARCH. IT IS QUITE FUN. DID YOU KNOW THAT I WAS THE ONE WHO CREATED WARIO'S CAR? WHAT? YOU DO NOT KNOW? DO YOUR RESEARCH!
[Source: The Laboratory Authority - Dr. Crygor, Page 21, Instruction Booklet of WarioWare, Inc.: Mega Microgame$!, May 2003]
This odd genius lives in seclusion. He built Wario’s bike. Mona’s moped, Kat’s sword, and Jimmy’s cell phone. He wears a cryogenic suit that doesn’t protect his head.
[Source: Dr. Crygor's character profile, WarioWare, Inc.: Mega Party Game$!, April 2004]
The Standard Kart series of builds was then introduced as a replacement for the Pipe Frame in Mario Kart Arcade GP 2 and carried into tournaments from Mario Kart DS onwards. As a result of this, in later tournaments, custom or pre-existing vehicles were allowed to enter the tournament upon being modified to be legal for the Grand Prix. The Pipe Frames did eventually return in tournaments from Mario Kart 7 onwards, which featured more in-depth kart customisation.
Speaking of which, the Mario Kart events are sponsored by other brands Mario owns, such as the aforementioned Mario Motors, Mario Star, etc. These sponsors, as well as the many others not otherwise connected to Mario, likely fund the events and even provide them with parts and supplies.
It stands to reason that Mushroom Piston Engines provides engines, Fuzzy Battery provides batteries, Bob-omb Plugs provides spark plugs, Dream Gliders provides gliders, 1-Up Fuel provides fuel, Bowser Oil provides oil, 100% Organic Antifreeze provides antifreeze, etc.
Mario Kart Arcade GP 2 makes reference to a Grand Prix Committee, which sends letters to Cup winners to invite them to the Cup’s special Challenge stage. Their letters are closed with an “M” stamped seal. This would imply Mario is at least on this committee, if not the head of it. This is presumably the committee that organises the events and decides which participants to invite, what items will be used, etc.
As mentioned previously, all of the participants either know Mario directly or have connections to people who know Mario. The Super Mario Kart manual seems to call attention to how each of the participant racers connect to Mario specifically.
MARIO & LUIGI
The Mario Brothers have pretty much the same abilities in cart driving. Mario and Luigi have had a friendly rivalry that goes back to their childhood.
[Source: THE 8 CART DRIVERS, Page 21, Instruction Booklet of Super Mario Kart, September 1992]
PRINCESS & YOSHI
At first, you might think that The Princess and Yoshi would have nothing in common (except their fondness for Mario).
[Source: THE 8 CART DRIVERS, Page 22, Instruction Booklet of Super Mario Kart, September 1992]
BOWSER & DONKEY KONG JR.
These two drivers have long been Mario's arch enemies.
[Source: THE 8 CART DRIVERS, Page 23, Instruction Booklet of Super Mario Kart, September 1992]
While Toad and Koopa Troopa's descriptions don't specifically connect them to Mario, Toad is of course a long-time friend of Mario and servant to Peach, while Koopa Troopa is a minion of Bowser:
Usually a faithful underling of Bowser, [Koopa Troopa] shows even the big boss no mercy during a race.
[Source: Characters and Special Items, Page 20, Instruction Booklet of Mario Kart: Double Dash!!, November 2003]
Koopa Troopa, of the Turtle Tribe. Usually one of Bowser's minions, but as far as races go, it's a different story!?
[Source: Characters, Mario Kart Wii, Nintendo Co., Ltd. website]
In short, Mario is what brings them all together - and in turn they each bring their own special items.
The iconic Banana Peels thrown by racers in the Mario Kart series are actually from the Kong's Banana Hoard on Donkey Kong Island, having originally been brought to the races by Donkey Kong Jr. back in Super Mario Kart:
Because of Donkey Kong Jr.! He likes bananas, and the peels are slippery, so they got the okay.
[Source: Tadashi Sugiyama, Director and C.G. Designer of Super Mario Kart, Nintendo Classic Mini: SNES developer interview – Volume 4: Super Mario Kart, September 2017]
In the original game, the only CPU-controlled character who throws banana peels is Donkey Kong Jr., which was a way to characterise the characters.
[Source: Hideki Konno, Director of Super Mario Kart, Nintendo Classic Mini: SNES developer interview – Volume 4: Super Mario Kart, September 2017]
Despite Donkey Kong Jr. not appearing in tournaments after Super Mario Kart, it seems his son, the current Donkey Kong, continued to supply Bananas from Mario Kart 64 onwards. Donkey Kong and Diddy Kong even brought along some Giant Bananas in Mario Kart: Double Dash!!, Mario Kart: Arcade GP 2 and Mario Kart Tour:
Special Item
GIANT BANANA
Only DK and Diddy know where these monstrous fruit can be found.
[Source: Characters and Special Items, Page 19, Instruction Booklet of Mario Kart: Double Dash!!, November 2003]
From this we can speculate about the origins of other items. Shells are common clothing/armour worn by Koopas, but seeing as the shells in Mario Kart are standard Koopa Troopa Shells, it stands to reason that the Koopa Troopa racer or perhaps Bowser brought the Red Shells and Green Shells originally used in Super Mario Kart.
These shells are worn by Koopa Troopas, both within and outside of Bowser's Army. We can even see one of Bowser's Airships in Super Mario Galaxy equipped with a shell dispenser:
The Green Shells and Red Shells even function how you'd expect - with Green Shells flying straight forward and Red Shells heading towards the nearest target!
However, with the expansion of Mario Kart in later titles the shells, which are mass-produced, appear to be provided by a company, with sponsors for "Green Shell Strike Equipment" and "Red Shell Strike Equipment" making appearances:
Koopa Troopa and Koopa Paratroopa seem to have their own supply in Mario Kart: Double Dash!!:
Special Items
TRIPLE SHELLS
(GREEN / RED)
Don't let Koopa and Paratroopa worry about not having enough shells.
[Source: Characters and Special Items, Page 20, Instruction Booklet of Mario Kart: Double Dash!!, November 2003]
Bowser also provides his own shells for the tournaments in Mario Kart: Double Dash!!, Mario Kart: Arcade GP 2 and Mario Kart Tour:
Bowser's Shell
This big ol' shell on loan from Bowser will crash any karts it hits and keep on going. It can only be thrown forward.
[Source: Items, Tips and Tricks, Mario Kart Tour, September 2019]
On the topic of items, Super Mario Kart used ? Panels to dispense its items. These panels are likely made by the same people in the Mushroom Kingdom who make ? Blocks. While they weren't used in later Mario Kart tournaments, they have still made a few other appearances since, such as in Mario Party 9.
From the Mario Kart 64 Mario GP onwards the ? Panels were replaced with the iconic Item Boxes, which were likely much more convenient for transportation and addition to the racetracks. But it wasn't until Mario Kart Wii where we actually learned where these come from, through the course Toad's Factory.
This is actually the Item Box Factory, as we can see not only from the trackside banners with the Item Box logo and giant sign of Toad with an Item Box, but from the shipping containers items ready to go into the Item Boxes:
Inside the factory, we can even see the process of how Item Boxes are made. The Items are put inside standard Brick Blocks, before the Brick Blocks are placed on a conveyor belt with stamps of some sort. These stamps appear to be filled with some sort of magical rainbow liquid, likely sourced from Power Stars, which transforms the Brick Blocks into the Item Boxes, giving them their translucent rainbow appearance and their ability to float:
Circling back to the Kongs for a moment, Funky Kong is actually the one responsible for the construction of the DK Barrel Cannons featured prominently throughout Mario Kart courses.
We learn this when Donkey Kong mentions Funky in the context of their creation in Mario Super Sluggers:
Oooook!
(Looks like Funky Kong has been busy.)
[Source: Donkey Kong, Best Friend of Funky Kong, Playable Character in Mario Super Sluggers, August 2008]
This isn't all too surprising though, given Funky Kong's previous barrel-based mechanic work and his appearances in Mario Kart Wii and Mario Kart Tour, as well as the barrel-inspired Funky Stadium which he presumably also built:
Speaking of which, it's about time I discuss the stadiums, circuits and other courses and where they came from!
Mario is world-famous and incredibly wealthy from his adventures, businesses and sports stardom. Multiple of the circuits in the games are also clearly built by him or at least in honour of him. He‘s the only character with actual circuits named after him in Super Mario Kart:
State-of-the-art racetracks in the Mario world that are rich in variety. The effectiveness of the pipe obstacles is the key.
[Source: Mario Circuit's description, Japanese Store Promo for Super Mario Kart, August 1992]
Mario Circuits have also been described as the primary circuits of their respective Grand Prix:
Mario Circuit
This is the main circuit in Mario Kart: Double Dash!! Familiar denizens of the Mushroom Kingdom can be spotted here and there. The road's filled with curves, so this is a good place to practice drifting.
[Source: Courses, Page 31, Instruction Booklet of Mario Kart: Double Dash!!, November 2003]
It stands to reason that most characters had a hand in designing the tracks that bear their names. In fact we know that's the case for several courses, such as N64 Wario Stadium:
That rascal Wario was in charge of the design and construction of this course. A fan of motocross races, he brought in major amounts of sand to fill this huge stadium in an attempt to build a course more suitable to bikes than Karts. Each lap's distance is extremely long, making it difficult to stay concentrated on the race.
[Source: MARIO KART 64 COURSE MAPS, Page 30, Instruction Booklet of Mario Kart 64, February 1997]
Meanwhile the Bowser's Castle courses are modified from or otherwise based on the real Bowser's Castle:
Surprisingly, the Bowser's Castle has been transformed into a race ground! Because it's inside the building, most of the corners are tricky right angles. However, the most difficult part of the course is where the Thwomps are. They will block your path, and if you run into one, you will spin out. Should you be stepped by one, you'll find yourself squished flat as a pancake.
[Source: MARIO KART 64 COURSE MAPS, Page 31, Instruction Booklet of Mario Kart 64, February 1997]
Bowser typically designs his courses with the help of his minions to be filled with traps in the hopes of beating Mario. This goes beyond the Mario Kart tracks and also applies to his Tennis Courts, Golf Courses, etc.:
Lord Bowser designed his course to defeat Mario, so it’s kinda full of lava and bombs and stuff like that.
[Source: Spiny, Minion of Bowser, Non-player Character in Mario Golf: Super Rush, June 2021]
Bowser enlisted his minions to create this course in hopes of defeating Mario with all the traps and hazards.
[Source: Koopa Troopa, Minion of Bowser, Non-player Character in Mario Golf: Super Rush, June 2021]
However, this isn't universally the case. For example, Peach Beach is merely Princess Peach's favourite beach resort in the Southern Isles, while Toad's Turnpike is on a public road in Toad City!
I could go on and on about the Mario Kart series and the depth to its worldbuilding, but I think that's quite enough for today. Thank you for reading!