Basically, on ATProto, everyone stores their own data on a PDS (Personal Data Server). This includes every post, image, video, follow, like, block, etc. The relay crawls the open web for PDSs to consolidate into a single stream of data.
From here, services can build off of that single data stream without needing to do any crawling of their own.
Of course, this does give the operator of the relay a lot of power. If they were to block your PDS, then only services relying on a different relay can access your data. This is especially why it is important that there exist independent third-party relays, but no one has taken up the mantle yet.
How? And the fediverse works the other way around.
Basically, on ATProto, everyone stores their own data on a PDS (Personal Data Server). This includes every post, image, video, follow, like, block, etc. The relay crawls the open web for PDSs to consolidate into a single stream of data.
From here, services can build off of that single data stream without needing to do any crawling of their own.
Of course, this does give the operator of the relay a lot of power. If they were to block your PDS, then only services relying on a different relay can access your data. This is especially why it is important that there exist independent third-party relays, but no one has taken up the mantle yet.