@lizziegadd Some servers have joining requirements, e.g. fediscience.org is for active researchers. Others have waitlists or are invite only.
However, you can follow & interact with people on other servers (if they're not blocked), so the experience is similar to birdsite for your main feed, boosting, replies. You can have accounts on different servers or swap server: you can take your followers (but not your toots) with you & the old profile directs people to the new one. [2/2]
@janneseppanen @lizziegadd It's how Twitter used to work, though without the specific communities - they developed hashtags like #ScienceTwitter to organise.
I believe your Home feed is toots and boosts by the people you follow, in chronological order. There's also a server feed, akin to a Twitter list for your whole server, and then the global firehouse across all servers that yours has federated with.