*Does a funny little dance* It is Vanilla Minecraft World Lore Time
To begin with, I wish to state that all of this is pretty in flux (haha) as I've only very recently gotten back into Minecraft and keep finding tiny details that make me 180° various peices of lore (the existence of dragon heads, achievement names, etc.). Feel free to ask questions!
I'm also going at this from mainly a more Attempting To Be Scientific view with added magic for spice.
The End is the beginning for our story, so we'll start here. Unlike many, I tend to lean more towards the idea the End has always been a very desolate place. The End is very low in oxygen with a thin atmosphere and low temperatures, thus the only creatures that manage to live here are either extremophiles(??? but non-microbial. I can't seem to find the word, F) or invasive species!
The only non "plant" life native to the dimension and still existent just pre-collapse were the Dragons, thriving with no natural predators and putting such a strain on the environment that others simply couldn't survive. They possibly evolved to become such a danger through less than natural means, perhaps due to radiation, or more magically, Void Corruption.
What would bring about the Collapse - Overworlders, specifically what I've nicknamed Raiders. The Raiders would manage to create a portal stable enough to transport in others and eventually lead to the over-hunting of the Dragons, as seen in End City Dragon Heads, Elytra (dragon wings), and the existence of Diamond Armor when no diamond ore is found in the End.
The Raiders, however, didn't consider that not managing to slay the Matriarch, one old enough to be more Void than End at this point and absolutely a threat, would risk everyone in the End and beyond.
The Nether is not exactly a homely place, and as I wrote grew to become the exact opposite of The End - high temperature, extremely high oxygen (allowing sustained infinite fires), and to a degree full of life (though just as extreme in some manners as The End).
Going by post-Nether-Update standards for this lore, this absolutely includes an incredibly diverse many-biomed Dimension. The dominant race of here was the Piglin and Pigmen, adapting through ingelligence similar to our world's humanity. They were craftsmen, making their own tools and even enchanting, and were potion brewers perhaps as well with the abundance of potion resources.
Unlike the End, the traveling between the Nether and Overworld was done with a decent bit more ease, assumedly due to less Void Corruption. This most likely lead to trade between the two, but I can't say that for sure as there are few remaining signs. There was certainly travel, however, as the Ruined Portals are gold-adorned, a sign of Piglin management.
And yet the achievement "Good Ol' Days" implies something felled the civilization. And with the Piglin being heavily decentralized for my HCs due to diverse biomes and the Nether not being the most practical environment for transportation beyond Strider and Hoglin use (slow and not too mobile), a full dimension-wide race dying out had to have been something huge.
The Overworld was average by most standards which makes it the most stable by far, with a livable amount of oxygen, its own night/day and seasons, and a growing centralized civilization.
Possibly the first to commit to dimensional travel, the soon-to-be Raiders traded first with the Piglin they would meet, and then moved on to attempt to reach The End (I'm unsure as to how they knew of The End at this point, admittedly).
They would reach The End with a significant amount of weaponry and armor in comparison to The End, equipped in Diamond and quickly creating their own bases of operation using what they could find and pushing back against the Dragons. Some dragons would assumedly escape, reaching back to the Overworld and dying due to over-oxygenation and heat and being risen by Overworld Corruption to become Phantoms.
But as the scales tipped to the side of the Overworld, The Matriarch, Queen of the End, would assert her authority and protect the main island, leaving a massive gap between her island and the rest of her world to protect her last remaining child. The Void would begin to corrupt the Raiders - creating Endermen - warping them beyond recognition, the portal would be shut, and Void Corruption (see: Warped Forests) and war (through use of newfound soldiers perhaps?) would bring about the fall of all other civilizations of the time, leaving a still living but silent world.
I have more but they're smaller headcanons on things like Piglin culture/politics, overall dimensional corruption magic ideas, possibly Endermen biology, etc.!