[k__]

Does this mean, I can use any AP implementation to federate with Mastodon servers?

[Kye]

As long as it implements the same activities. They often do.

[ryukafalz]

Roughly yes, though you'll want one that supports the same activity types (Mastodon mostly uses "Note") or it won't make much sense :)

[k__]

How many types are there and how prevalent are they?

I read some websites about "host your own server" and some were talking explicitly about Mastodon and some were like "let's do selfhostes microblogging with this AP server"

And I asked myself, if I could use AP servers to follow people and let them follow me when they're on a public Mastodon instance.

[Matl]

Yes, abet Mastodon has some non-standard extensions. But basically yes.