Part 2:
If you are referring to Qt documentation for libraries, functions, etc, then I think the best place is here:
doc.qt.io
. Although I will say that when I have a question about an object or method, such as if I am working with a QLabel and I'm wondering what methods it has, or descriptions of its methods, or examples of using it, I just do a google search. Usually the google search comes back to the QT docs, but not always.