Each server (instance) is its own thing with its own rules and logins, but they all share the data of communities and posts when those are subscribed to by outside instance users, unless one blocks the other (defederated). Think of it like email, different clients sending each other email, and a few might be flagged as spam and not seen. So to your point, you only need to login to one of the places and just subscribe to others outside to see them in your feed.