Now I’m not quite sure you understand where I am getting at: I’m trying this another way.
(1) iPhone apps in the iPad are executed, handled, and controlled as if the iPad is a huge iPhone.
(2) It means: The orientation of the iPad will then orient the app as it would be on an iPhone.
(3) the app responds according to how it is designed (or programmed)
(4) the app then May not have a landscape orientation.
(5) the app then appears in portrait mode sideways, when the iPhone or iPad is turned on its side, I.e landscape.
(6) in an iPad: as it is mounted to a magic keyboard, so as to make it function more like a laptop, an iPhone app is in portrait mode still, laying sideways.
(7) as an iPadOS update, there should be at least an option, that the iPhone app in question, can appear in portrait mode, scaled accordingly, and positioned upright while the iPad is in landscape mode.
(8) this is independent of how apps are designed by third parties. The apps can behave exactly as if it was just designed for the iPhone. iPadOS will be aware however on how to handle iPhone apps in the iPad Pro for example.