I see Mastodon being mentioned a lot (and sometimes Pleroma) as if that is the only application to access the fediverse.
ActivityPub is still in early stages of adoption. It is great that these apps have proven themselves in production (with a couple million users) based on a v1.0 of the spec where intentially a lot of things were left out, like authz, account discovery, search, etc.
Right now, based on experience and with a good base of knowledgeable AP devs, the spec is evolving. With its flexibility and JSON-LD at its base federated applications can be created for any number of domains.
And this is happening. Almost every week there are new project announcements that adopt AP in some way. They will not all federate with Mastodon from the start, and there are many challenges to be tackled to make a seamless fediverse integration smooth sailing.
Here is a good AP project watchlist: https://git.feneas.org/feneas/fediverse/-/wikis/watchlist-fo...
There is an online AP conf in October for those interested: https://conf.activitypub.rocks/#home
For questions you can go to these forums: