Having a quick look through their repos, it looks like they don't use Haskell for the mobile platforms?
> cpp-for-mobile > Template for cross-platform mobile app with native UIs and C++ core
> cpp-for-mobile > Template for cross-platform mobile app with native UIs and C++ core
Their flake.nix indicates they use Haskell to generate cross compiled shared library for Android, iOS, Mac, Linux and Windows.
I am not expert in Nix but at high level I can see they are indeed using Haskell.