Qt - library. It's not.
Divided into components for network, XML, JSON, SQL databases, OpenGl, sound, and whatnot.
There are components to generate the visual interface.
In Qt there are 2 components to create UI Widgets and QML.
Widgets was the first. Then, when it was necessary to meet the requirements of time, came up with QML.
But widgets are outdated, were not bad. They are easier to understand.
Decide the task. What interface you require. Ask at the Toaster, can it be done on Widgets. Google interfaces created using the Widgets, maybe you want.
QML for a beginner is not recommended.