Fediverse

It's not, they have their own system.

AFAIK, Bluesky started on ActivityPub at first, but then it was decided to make a new protocol which resulted in Atproto. It also started as an internal project at Twitter, was funded by Jack, but then as it got popular amongst a more regular audience, he left when he kept getting pestered with @ mentions and anti-crypto stuff. He hangs out at Nostr now and from what I've been told, isn't really involved in Bluesky's meetings.

There was an effort to bridge Bluesky/Atproto, ActivityPub and also Nostr together - Bridgy Fed - for when Bluesky started getting their protocol federating outside its own network. The issue was, the creator made it opt-out rather than opt-in. The AP fediverse collectively shat themselves, spreading their delusions about Bluesky, one guy called the creator a rapist for using public data and another threatened to sue/fine the creator. It was absolutely bonkers and that incident exemplifies a good part of why people find the fediverse to be toxic, moreso than anything involving Threads.