Indeed, an Uno app is a native app, it uses UIKit on iOS, Views on Android, HTML on Web, except on Linux where you only have access to our Skia renderer.
Yes, on Linux it behaves similar to Flutter and draws directly to a Skia Canvas.
It used to have a dependency on GTK but no more since the 5.2 release.
There's just no notion of "native controls" that you can embed in an Uno Linux app like you could have if you wanted to embed a 3rd party native UIKit control on iOS.