"In 2017, the Qt Company estimated a community of about 1 million developers worldwide[18] in over 70 industries."
See list of companies using C++ QT for GUI,
See list of companies using C++ QT for GUI,
2 - Qt has been migrating away from pure C++, again you also missed pure from my comment, modern Qt applications are written in Qt Quick, a JavaScript dialect, with underlying components written in C++.
C++ Widgets have hardly changed since Qt 4, other than being updated to the underlying Qt infrastructure.