That would be the "old fashioned" option, or what their site seems to call Qt GUI, and not compatible with Qt QML or Qt Quick (or Qt Quick Controls 2 which I think are just an upgrade)... I think.
QML and Quick are basically the same in colloquial use. Technically QML is the language and Qt Quick the UI system built on top of it. Qt Quick Components (2) are some higher level building blocks for UIs.
It isn't widely used yet (as far as I know) but there's a library to use QML as the language for building QtWidget-based apps:
It looks a lot like SwiftUI, although I know they work fundamentally differently. I wonder if Qt could adopt that approach somehow, maybe with new C++ features, or maybe by adopting Swift...