UWP is most definitely not HWND per control, and doing so would be incredibly limiting. I don't know the UWP (WinUI) internals, but on mac it's generally a CALayer per widget, and Core Animation is doing the compositing. On Windows, the Windows 8 equivalent is DirectComposition, but I think the WinRT Windows.UI.Composition interface has some capabilities not exposed through winapi.
It isn't HWND per control indeed, it does its own composition, hence why i mentioned Swing and JavaFX with an "especially the latter" in parentheses (but Swing also does bypass the underlying window system aside from the toplevel stuff for per-control functionality whenever that'd make sense).
