What’s the difference between all those instances?

We have Lemmy.world, Lemmy.ml, beehaw.org, I think even yiffit.net. If I log in using wefwef it asks me where I want to login. And when I make a new account using another instance, it says ”… Lemmy is federated, so you can interact with everything on lemmy.ml even if you’re registered on a different instance.“ So if...