It's now possible for one application to access files that belong to another, but there are some security restrictions which make application platform portability difficult. So it is not clear yet what needs to be done to make the best use of QFileDialog or QtQuick.Dialogs FileDialog in an iOS application, or if that's even a suitable paradigm. But the document picker on iOS does actually look like a file dialog, and it's interesting that it also provides access to content stored in iCloud:
At this time it seems like one way forward would be
1) implement QPlatformFileDialogHelper for iOS, to use UIDocumentPickerViewController
2) augment QFile to call startAccessingSecurityScopedResource only when necessary
or maybe that will turn out to have unavoidable pitfalls.
There are also older ways of sharing files between applications, usually by accessing the content in its "owner" application and somehow "sending" it to another application. Those don't map as well to the use of a file dialog, but maybe we need some sort of Qt API which works at least across iOS and Android, or maybe interoperates with drag-and-drop on desktop platforms, or something like that.