How does federation work with the "subreddit" model?

Hi, I'm new here, so sorry if this has been asked before.

I understand that Lemmy is supposed to be like a decentralised Reddit, but I've got a question about this; on Reddit, there is only one server, with subreddits on it, so there is only one r/Music, only one r/AskReddit, etc.

However, on Lemmy there are many servers; would this lead to a situation where many servers have repeats of the same popular communities or does this not happen in practice? Is there a good way for me to find communities across all federated instances? Thanks :)