There is a whole lil ecosystem of apis that the @pixelfed app uses, this one for example, provides a list of android app build releases for use on the https://pixelfed.org/mobile-apps page and also in the app itself (so users can easily self-update in-app)

That being said, sometimes they add more complexity and problems and need to be re-evaluated like how the app no longer checks login/register instances against a list of known instances as it lead to users of newer instances not being able to log in.