WinUI 3 still supports WinRT. It ALSO supports more. It's an evolution of WinUI 2, not just a simple version bump, but also not a completely new tech. It's probably a closer evolution to go from WinUI 2 to 3 than it was to go from Angular 1 to 2.
https://learn.microsoft.com/en-us/windows/apps/winui/winui3/...
https://learn.microsoft.com/en-us/windows/apps/develop/platf...
> WinUI 3 still supports WinRT.
I think this is completely independent. You can simply use WinRT APIs, because Win32 Apps can use them now. WinUI3 apps are win32 apps.
> but also not a completely new tech.
Not sure about this. UWP APIs work out of the box. For WinUI3 you need the Windows App SDK, and it is much slower and heavy than UWP (out of curiosity I created a very simple app and it was fast just a few dozen kbs big)