app.xaml

XAML markup file Reference

A .xaml file contains user interface markup in XAML, the Extensible Application Markup Language. Developers declare windows, pages, and controls as XML elements. Frameworks such as WPF and WinUI read the markup and build the interface objects. The design stays separate from the program code. The build can compile the markup into the binary .xbf form.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.