I can only speak for myself, but as a complete newbie who wanted to create an account and figure out how lemmy works, I honestly thought lemmy.world is the main site and everything else is just smaller niche communities.
There was an ELI5 post that clarified how it all works, but that was after I created my account.
Yea I made like 4 accounts with .world being the last, because I thought I needed an account for every instance. I'm now learning I don't need to do that.