Does anyone know what the UI of Edge is written in? Since it runs on Mac as well, have they ported some of the .Net UI frameworks to Mac or is it native all the way?
Edge (and Chromium) both use a custom C++ UI framework. Chrome wrote the framework to be cross-platform, so Edge was able to take advantage of that to deliver the macOS version. AFAIK there's no WinUI XAML components being used.