Bluesky currently has the only active PDS, and is acting as registry for all its accounts. In theory, they'll open that up to other registry providers. For anyone who already runs a website, the simplest thing would be to bundle DIDs with their current service. And maybe that's how it'll shake out—although, it's worth noting that Bluesky uses their own DID schema, rather than the one developed by the W3C. So maybe they'll end up being the central registry for all accounts on the network!
Okay, so the PDS handles personal interactions, and your verifiable identity is stored on a (currently centralized) registry. What else? There's a set of account features the protocol lumps under the heading of "big world" networking, which includes "large-scale metrics" as well as algorithms and search. Those are handled by indexing services that crawl the network collecting information. Basically, Bluesky will be running its own Google for the whole network.