Mastodon uses an open API for its servers, which means that anyone can write a Mastodon app and it can make full use of all of Mastodon’s features. This openness means third party Mastodon apps get just as much access to Mastodon’s features as official apps do, and many third party Masto apps are actually better than the official one.
However, a side-effect of this openness is that non-Mastodon Fediverse platforms can also make themselves compatible with the Mastodon API. This means that Mastodon apps will work with those non-Mastodon Fedi platforms too. For example, you can sign in on a Mastodon app using your PixelFed account.
Not every Fedi platform supports this, but many do. The easiest way to find out whether it works is to download a free Mastodon app and try signing in.
(It should be noted that Mastodon’s developers don’t officially support this, and if it stops working you need to check with the maintainers of your Fediverse platform. They probably need to update their software so that it remains compatible with the Masto API.)