Worldbuilding is a massive undertaking.
You're responsible for figuring out who and what exists, how days are spent (and how long is a day, anyway?), and what these beings believe about everything from religion to politics to pineapple on pizza to medicine and everything else under the sun(s).
Now imagine doing that for an entire planet.
Yeah. My head just exploded, too.
Looking at some of my favourite fictional universes, and a few others along the way, it feels like one of the best ways to keep things more manageable (and less "the Big Bang is throbbing inside my head") is to break things down. One step at a time.