Sometime pre-iOS 26 it was possible to use a SplitViewController so that on iPhone you saw the master as a side menu (e.g. hamburger), and the detail full screen. While on iPad you would see the master displayed in a sidebar that could be closed, with the detail fullscreen.
This has changed, using a SplitViewController on iPhone now forces you into having 2 fullscreen screens, with a push/pop layout and a back button. In situations where you want the detail screen to open first, this results in users opening the app to find a back button already presented, despite having not navigated anywhere. They must "return" to a screen they never saw. I really despise this layout and find it to be quite a UX issue. But given the new iPhone Duo, using the SplitViewController is now one of the easiest ways to maintain the necessary responsiveness needed, forcing us to accept this behaviour on regular iPhones.
Can we PLEASE get the old functionality back, where we can explicitly state on iPhone that the master will always be displayed as a menu?