So, I think I'm starting to understand this now. Mastadon is a software built around the ActivityPub protocol but there are other softwares such as Friendica that use the same protocol with different style of layout and function. What other softwares use this protocol ?
A lot! One neat feature of ActivityPub is you can build all kinds of stuff on it. Mastodon is more of a Twitter "clone", Friendica/Diaspora/Scuttlebutt are Facebook clones, and PeerTube is a Youtube clone etc.