If you want to be "normal", or at least follow the Microsoft semi blessed path, and make an appx, that does not include WinUI3. The user installs it, and in the process pulls down WinUI3 (also an appx) if it is not already installed. It's only if you really insist on "self-contained" that this problem arises.
I say semi-blessed because of course Microsoft aren't really committed to WinUI3 and barely any of their own apps use it.
If you want to be "normal", or at least follow the Microsoft semi blessed path, and make an appx, that does not include WinUI3. The user installs it, and in the process pulls down WinUI3 (also an appx) if it is not already installed. It's only if you really insist on "self-contained" that this problem arises.
I say semi-blessed because of course Microsoft aren't really committed to WinUI3 and barely any of their own apps use it.