In so many words and with usual way to make me feel like an idiot for asking I now have BETTER understanding of widgets.
Thanks.
Especially the structure of QMainWindow.
Simple - it contains three QWidgets and the one I want to work with is the "centralwidget".
I have NO issues adding to "centralwidget" using QtDesigner GUI, i will be looking into HOW to add generic QWidget in code.
By generic I mean simple "button" or entire class - such as QDock.
I'll ask for more help IF I NEED IT.