Btw when you run a PDS, it just just proxies all requests to api.bsky.app. The PDS exposes HTTP endpoints on its own domain, and when you use apps they do hit your domain, but the PDS itself just proxies the requests to the Bluesky centralized server at api.bsky.app.

Your own posts do get saved into a local SQLite database, and there's an endpoint where other servers (like Bluesky's giga relay) can index your posts. It gets away with SQLite because it literally only stores your own posts. Feeds, other people's profiles, etc all get fetched from the upstream in realtime.