Also notice that Lemmy is actually multiple sites connected together, so there might be more than one of each community, which can be confusing at first.

Nope. Best thing to do is browse all the posts on all the communities for a while and then start subscribing to the communities you think you might like. There is also https://lemmings.world/c/newcommunities@lemmy.world for new communities to advertise themselves.

What’s your definition of main community?

No thanks