As an aside: Expect ads. The functionality of Bluesky depends in no small part on big index servers that constantly crawl and store data about every user, regardless of what PDS they're posting from. Those index servers will almost certainly be run by for-profit companies, and they're going to want to recoup their costs somehow. They may not serve ads directly to your timeline (though, they handle the algorithms, so they surely could), but they'll serve them to you somewhere.
Presumably likes, reposts and follows are also stored locally, in the data repositories for whatever PDS you're on, but for our purposes, that's entirely redundant, because they're also being indexed on these huge, centralized servers. In fact, one way to make sense of the idea that you can restore an account even when its home PDS is no longer available, is to suppose that it's being restored from an index, like pulling up a webpage archived by Google or the Wayback Machine.