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.
To be clear, I don't know if you'll actually be able to restore an account from an indexing server. The protocol doc assumes you'll do it from your CLIENT. Which means that the app you post from is expected to make a persistent backup of your entire account—"contingent on the disk space available." So if the idea of being able to port your entire post history around is a big selling point for Bluesky, expect to eventually have dozens of gigs of account data riding around on your phone.