latest qt-unified-windows-x86-3.0.5-online which C compiler It can use?

My new qt installation creates QCreator .. which is using which compiler?. I do not have option during installation which compiler is going to be used (selected), for instance MinGW as it was provided before or to use MSVC 2017 (maybe at the same time both as an option)?
Even when I go to visual studio and from there to QtVStools, and then QToptions and select the folder where the latest qt installation is located, the system is complaining it could not find qmake file.
Maybe I have stuffed up with reinstallations of qt?

I tried to uninstall qt, and then to install it again, but it is the same as before.
Maybe I had to delete something after the usual procedure of uninstallation?
Do I need to install MinGW beforehand to use its gcc compiler?
if I do whether it is 32 or 64 bit compiler? I am on 64 bits machine.