How can you tell / identify which macOS Recovery actually boots with a dual macOS installation in one APFS partition/container?
This is 2013 iMac with High Sierra and Catalina installed in the same APFS container. When first booted to a recovery you will a small icon next to Reinstall macOS, which represents which macOS is install. Below is High Sierra.



Next is Catalina.



By selecting macOS Utilites-&gt;About macOS Utilities from the menu bar, you can get a larger icon. Below is High Sierra.



Next is Catalina.



By selecting Reinstall macOS from the initial window, you can get an even larger icon and a label. Below is High Sierra.



Next is Catalina.



By selecting Utilties-&gt;Terminal from the menu bar, you can get Terminal application window. If you enter the command sw_vers you will the the version of macOS. Below is High Sierra.



Next is Catalina.