Is this design correct?
System Qt 5 has qtvirtualkeyboard thich is GPL.
Can I link my proprietary app with system Qt?
Can I create open source app which run Qt Virtual Keyboard then load my app as module?
The GPL is intended to cover the whole program. You cannot create a proprietary program that loads a GPL library. Where a GPL program loads a plugin, that plugin would be subject to the GPL as well.
The GPL FAQ addresses these questions in more detail:
There is one caveat: these issues are not clear from copyright law, and there is no court precedent. This answer presents the Free Software Foundation's opinion on this matter. It is also possible to argue this differently, in particular that the GPL cannot extend to the entire program. But if you want to stay on the safe side of this grey area, you should assume that the FSF's interpretation is correct.