Qt quick for beginner

[wysota]

Could you elaborate in which case it is useful for desktop app?

If you're wondering whether it is useful for your particular app then probably it's not

And seriously, if your app is mostly about processing data and little about presenting it or presentation needs to be very complex then QtQuick probably won't be much of help. If your app is mostly about (simple) user interaction then using QtQuick can reduce time-to-market significantly.

And what is the architecture of a common qt quick app? I find qt apps adapt Qt Model-View architecture, does a qt quick app stick with a particular architecture too?

Qt apps can adapt model-view architecture but they don't have to. It's exactly the same with QtQuick apps -- they can follow model-view-delegate architecture but they don't have to if your system is not about handling data. Usually for a complex app you would write the data backend in C++ and expose it to QtQuick where you could implement the frontend with QML.