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?
No. For each Mastodon, Lemmy, or Peertube server you want to join, you need a separate account on the server.
Not true. You can follow Peertube from the other systems. You DO NOT need an account on each. Is it easier to use the platform natively? Yes. But you can follow threads on Lemmy from Mastodon and what not.
I think it’s reasonable to assume that when OP asked about “joining” each platform, they wanted to use each platform natively.