"In case of dynamic linking, it is possible, but not mandatory, to keep application source code proprietary as long as it is “work that uses the library” – typically achieved via dynamic linking of the library."
However, one thing to note: the OSS version of Qt uses the LGPLv3 license, which has additional restrictions (like the "Anti-Tivoization" clause) which make it incompatible with the iOS AppStore thus forcing you to use the commercial version of Qt in those situations. Not sure about the Mac and Windows 10 app stores, I am curious if anyone knows/has experience with LGPLv3 and those stores?
[1] https://github.com/chromium/chromium/search?q=%22kde.org%22
The use of LGPLv2 on the iOS AppStore seems to be controversial. But nothing changes with LGPLv3 in that respect as far as I know.
Not necessarily. If you're able to distribute versions of your app to iOS users so that they can link it against their own Qt libraries, you'd be in compliance with the LGPL.
I've seen companies that distribute via the App Store, but also provide object files with instructions to link them against user supplied Qt libraries and to get them on iPhones/iPads.