Be aware that Qt's widget library isn't exactly well-suited for mobile usage and mobile apps usually use QtQuick with frameworks like Kirigami.

PureOS Amber unfortunately has a too old Qt to be able to say that it's a "first-class citizen" there :( It's going to change with a coming version that's based on Debian Bullseye though. Meanwhile, Qt apps run very well in Flatpak on Amber already.