Things are fragmented I’m afraid, Daniël. There’s many independent initiatives, and more starting every day. This SocialHub forum in itself constitutes a huge archive (but chaotic, as they are discussions). The FEP process is most organized, and the W3C SocialCG is being revived, right now. I am hoping people are open to bring more cohesion into all this.
The SocialCG mailing list is a good one to subscribe to. And the Fediverse Developers chatroom on Matrix. That wiki is long dead, and the lists moved to the fediverse organization on Codeberg. Also note the pinned Guide for new ActivityPub implementers … this is a wiki too, that anyone can update and help improve.