Disclaimer: I’ve only been looking into this for a few days, so I may say something incorrect.
My understanding is that there is a single Mastodon implementation, but there are a number of similar projects which can interoperate by sharing data via ActivityPub to become federated. Some of these other projects implement all/most/some of the Mastodon REST API so that GUI clients (phone apps, etc) can be used for reading and posting. There was an article on LWN recently which listed several of these with some commentary [1].
My understanding is that there is a single Mastodon implementation, but there are a number of similar projects which can interoperate by sharing data via ActivityPub to become federated. Some of these other projects implement all/most/some of the Mastodon REST API so that GUI clients (phone apps, etc) can be used for reading and posting. There was an article on LWN recently which listed several of these with some commentary [1].
[1] https://lwn.net/Articles/916154/