The landing page is shown if QmlDesigner is disabled and the user opens .ui.qml file.
The landing page informs about Qt Design Studio and shows various options. A concept is attached.

Opening a .qmlproject file should also move the user to the landing page.

the checkbox is: Always Open with Qt Design Studio. (Whenever a .ui.qml file is opened we launch QDS instead. This requires a .qmlproject file that will also be opened)

If the checkbox is enabled we should not go to the design mode anymore and showing the landing page, because it would become annoying.
If the checkbox is checked (which is not the default because we want the user to see the landing page at least once) the design mode has to entered explicitly.