Activity Pub: Can I join a PeerTube or Mastodon server using a Lemmy account?

Hey, I am currently trying to understand a bit better what the Activity Pub allows and what it doesn't as I am preparing a presentation on that topic.

On Lemmy I can join other servers without having to recreate an account. Does that also work for across Activity Pub supporting software? Could I join a PeerTube page and then post there? Could I upload videos? Could I join Mastodon or Pixelfed and post images?