Having used it since Windows 8 days, I know pretty much how it all goes.

Nowadays there are two WinRT models, the original underlying UWP that grew out of the UAP / WinRT evolution introduced in Windows 8.1, to simplify what was originally split across phones, tablets and desktop.

And now the WinRT implementation on top of Win32, started as Project Reunion, rebranded as WinAppSDK alongside WinUI 3.0.

I’m not trying to be particularly pedantic but that still doesn’t make WinAppSdk built on UWP; it’s mainly an expanded and cleaned-up collection of first-party cross-language wrappers/bridges/ffi to WinRT to hide the COM underpinnings plus unify some of the disparate Win32 vs WinRT APIs.

As you know, WinRT predates UWP. UWP as tech isn’t strictly defined but it includes things that are out of the scope of WinRT itself and aren’t available via WinAppSDK even now that UWP is finally, officially dead.