I am in the process of building QGIS from source (https://github.com/qgis/QGIS) for Windows 10. I am using QtCreator 7.0.2 based on Qt 6.2.3 (MSVC 2019 , 64 bit).
Does the current source for QGIS support Qt6?
Porting QGIS to Qt 6 is a work in progress as you can see in this QGIS Enhancement Proposal.
A lot of developments, tests and workflows are made for building QGIS with Qt6 but at this current time, it's not available.
If you want to go further with this question, I advise you :