Top Down v. Ground Up Worldbuilding
There are two main styles people use when building worlds: Ground up, and Top Down.
Ground up is where you start with a small location- Typically a dungeon or town. Then you flesh that location out with possible NPCs, things of interest, history, etc. Then you figure out how things within that area relate to the nearby surrounding area, and then continue doing this process with ever-larger details until you have created the kingdom, continent, or whatever you think the largest relevant area would be.
Top down is the exact opposite. You start with the entire world (or even Realm) and figure out what the big details are- major kingdoms, continents, gods, etc. and then figure out how those relate to smaller things.
In short- Ground Up is creating small details and figuring out how those relate to a broader scale while Top Down is having a broad idea and working down to the finder details
While both of these require the same amount of work to complete a world, there are still advantages to both on a micro scale, and you may find that you prefer using one or the other overall.
Ground Up is very good for oneshots and small campaigns. Unless your games are sandbox or high in improv, you may not need larger details at all- just a small contained area for your players to explore for a few sessions and some defined conflict in that area. If you stay focused on a small scope, it is much easier to determine what you might need to know about that area ahead of time.
On the other hand, I have found Top Down to be a very good method for longer campaigns with overarching story lines. While the story line itself should change based on what your players do, mentally putting your focus on the big picture as a DM can make it easier to realize what could be happening in the world outside of your players.
At the end of the day though, a lot of this does come down to your particular preference as a DM.