@Blaze AT (Bluesky's protocol) is a little bit different then activity pub. There's two types of servers, a PDS and a relay. A PDS is basically a git repository of all your posts/interactions, it's super lightweight and doesn't do anything but host them and provide it to any server that asks for it. The PDS basically does the profile hosting portion of a Mastodon server, and is very similar to a Nostr relay if you're familiar with that.
A relay accesses data across a bunch of PDSs and provides it as one big network to the relay's users. It's basically the equivalent of the federated portion of what a Mastodon server does. It's also doing what a Nostr client does (although Nostr does that on the user's device) if you're familiar with that.
Any relay can pull data from any PDS, so theoretically it's very decentralized since anybody could host either a PDS and/or Relay. Bluesky was opened up very recently though, so there's not many non-Bluesky-hosted PDSs on the network yet and most are small and experimental. There's also no relays other than Bluesky that I'm aware of, although it's only been open for ~6 months so I expect that'd change soon.