You get all future posts and comments posted to that community, you don't get a full historical backfill.
Posts and comments from users on instances that your instance has defederated with. Like sh.itjust.works has defederated lemmygrad.ml, so you won't see any posts or comments from users with an account registered there even though I'll see both of your posts/comments since my instance federates with both.
When you first subscribe to a new community that no one else on your server has subbed yet, it can feel weird that the comment history isn't backfilled. You'll see seemingly empty posts that have comments when viewed from other instances. This situation will right itself after a day or two as posts made after you subscribe will have their full comment history replicated to you as they happen. It's just the initial wave of posts that have a weird comment history due to the lack of backfill.

You don't have to follow user accounts at all. I'm not even sure if that's possible, I haven't done it.