wiki.qt.io/Qt_for_Beginners needs to be retired or rewritten.

wiki.qt.io/Qt_for_Beginners was applicable back when Qt 4 was new. Now, all it does is serve to confuse new authors. The fact that its banner states that it's applicable to Qt 4 and some Qt 5, whilst we're on Qt 6 now, demonstrates this:

This tutorial series target mainly Qt4. Even if most of these tutorials are also valid for Qt5, the case of Qt5 is discussed in a separate part.