"#ActivityPub, the ActivityStreams-based Social Media protocol that powers things like Mastodon, Peertube, Pixelfed or Misskey has a relatively simple model where the user profiles are federated directly between other servers in an explicit model
#ATproto takes a more modular and generalised approach to user data storage that is not just intended for "global" microblogging (or whatever other specialised AP software you run, e.g peertube for videos)."