The problem is Google added the option to Android (starting with 11) to declare some cameras as system cameras. Those can only be accessed by apps installed in on one of the system partitions (so basically shipping with the OS) or by apps signed with the system certificate (so by Fairphone in this case).
I don’t see any reason this option exists other than completely anti-consumer behavior, but it’s there and Fairphone are using it for the wide-angle camera for whatever stupid reason 
Haven’t tried it, from what I read it only supports wide-angle and maybe some other options, but certainly not the full feature set that would be possible with the stock FP Camera app.
And it’s installed as a system app with the necessary permissions, something you can’t easily do with 3rd party camera apps…
There are more problems when it comes to the advanced functionality I don’t understand either, @xblax will have to break that down, but that’s basically the gist of it.