It's the ActivityPub API (client to server) where clients supposed to interface with so that you could have one client, for all fedi. But turns out server implementations (Lemmy, mastodon etc) find it not so well designed or performant or adequate (or idk what else) so instead they implement their own client APIs, effectively making clients being specific to the server rather than interoperable (one client that could understand all servers). It's reasonable though, it's just .. I thought... that this was already done and established.. but well..