XAML, which stands for Extensible Application Markup Language, is a declarative XML-based language used primarily for defining graphical user interfaces in various Microsoft frameworks and technologies, including WPF (Windows Presentation Foundation), UWP (Universal Windows Platform), Xamarin.Forms, and WinUI.
Yes, you can use C++ to implement WinUI 3 apps. WinUI 3, being a part of the Windows App SDK, supports development in both C++ and C#