@pervognsen @rovarma
WinUI 1 was a controls library for UWP that was used internally within the Windows division at Microsoft on some of the built-in apps in Windows 10 for a while. It was never released to the public or even discussed publicly (prior to the public release of WinUI 2) AFAIK, so it's no surprise you didn't know about it.
WinUI 2 was an updated version of that controls library which was released publicly as an open source project.
Confusingly WinUI 3 comprises not only a further updated version of the WinUI controls library, but the entire UI framework underneath it including the layout and composition layers, all of which were forked from their equivalents in UWP XAML and "lifted" out of the Windows codebase. So it's not only a newer thing but a different category of thing than what WinUI 1 and 2 were ...