WinUI, WinAppSDK are built on top of WinRT, which is C++ for the most part.
That is the whole joke about it, Windows division tried to kill .NET with their updated COM based on Vista victory over Longhorn, and the whole AddRef/Release makes it slower than WPF applications.
https://arstechnica.com/features/2012/10/windows-8-and-winrt...
I think Groove Music/Media Player is C# though
If it is using WinUI is it as much C# as using doing AI in Python is Python.
See WinUi and WinAppSDK repos on Github.