@Are0h so, I think for a lot of people, the "Fediverse" feed on the Mastodon UI is "the fediverse". And, for another group of people, it's their main interface.
All of which is to say: to me, the fediverse is open, and what's powerful is that we can each control the experience we have.
But that's kind of liberal and individualist. I think a lot of people consider the fediverse to be a literal single feed, collectively managed, kind of in a communalist or syndicalist structure.