Imagine a bunch of cities run by different mayors. They all run their own cities but people can freely travel between them, go to the restaurants, visit friends, and more. If one mayor goes insane the other mayors can ban citizens from visiting their city. But there's only so much power one mayor has since they're under the state government.
Congrats, you now understand federalism. Idk how Lemmy works either.
Imagine you have a mutated horse. It has many heads. You can only see one. You can pick which one you see. But whichever head you see will tell you what the others are like, what they're doing and saying, etc. So you ride your horse, it looks like a normal horse, but secretly it has many heads, but to ride it you have to talk to the head that knows you. You could introduce yourself to the other heads but maybe they won't be as nice as the head that you know. When the horse runs, it sees with all of its heads and talks to you in a language that reflects it's thousands of perspectives.
it's not really necessary to fully understand the concepts behind lemmy, it's just like reddit when it comes to usage, but not everything is visible right away, sometimes you need to look up and subscribe to a community for it to be synced with this instance here and it'll show up. There is also https://browse.feddit.de/ to discover communities. Oh and if you clink on a link that points to another instance like lemmy.ml or lemmy.blahaj.zone it'll basically log you off, so don't do that, just use links that look like [email protected], for example. All that is left is to enjoy the experience :)