Using the lastest developer betas both on the Mac and iPhone I was looking into the iPhone Mirroring App and then tried to Fill the window and, to my surprise, the aspect had changed to the “tablet-like” view I guess we will have on the foldable iPhone.
Here’s a Notes app screenshot
I’ve found this to work only on first party apps (tested Safari, Calendar, Health and Mail) but not on 3rd party ones.