The idea of federation is that you have multiple instances of the same medium of social network; of the same lexicon. The same ATProto relay/firehose is the same instance of a social network. As explored in the link I gave you, ATProto does not make mastodon-scale self-hosting of relays easy, nor does it intend to, emphasis on “mastodon-scale”; instead, it makes centralized monolith self-hosting of existing/exported exoduses easy.