[captainmuon]

I would call it almost, but not quite native. There are some things MS internal apps can do, but that WinUI 3 can't. (Until recently, creating additional Windows, but also things like blurred transparency.)

I think a real native solution would be shipped with the system, and get an updated theme and feel with an OS update. But the last version that did this was Win32/Uxtheme and to some extent WPF. Metro/UWP formally did this, but it encourages you to hardcode a lot of styles, so you have to update your app when a new Windows version comes out. But I think MS has moved away from shipping the UI library with the system.

The real native UI is what MS uses internally, and for a lot of products that is DirectUI. It is used in Explorer, the start menu used it for some time in Win10, I think the control center also used it. But also MSN messenger used it, and Office, too. Spiritually it is similar to WinUI 3 I think: implemented in native code, drawing "windowless", and using some kind of XAML.

[fassssst]

DirectUI was a precursor to XAML and is being rapidly replaced by WinUI internally. It has janky markup and renders using non-GPU accelerated GDI.

WinUI 3 intentionally decoupled from the OS so app developers can support several versions of Windows while still using the latest GUI stuff, in the spirit of web apps.