The “fediverse” are distributed online social networks using a thing called activity pub. Because they all speak the same protocol they can interact with each other. This is why Mastodon and Lemmy can talk to each other.

BlueSky uses something called AT. AT proto has three things to it where . The pds (I think it’s called) is where you can choose to store your data. Relays aggregate those. Then you have to have a way to view it. In activity pub the first two layers are the same thing and we typically call them “instances”. Running your own relay for AT is wildly expensive and to my knowledge nobody is doing it because of that. Not enough people use their own pds to store data to even really make it worthwhile. The vast majority of BlueSky users are actually using BlueSky itself. Even if you consider the fediverse to be things doing federation, BlueSky isn’t really as federated as they lead you to believe.