Generic back button is not a thing in Phosh - so there is none.
Can you still do it somehow? Maybe if you have good understanding what that generic back button would do in situation x,y and z.
Inside Phosh you can use and run Lisgd (libinput synthetic gesture daemon)
https://git.sr.ht/~mil/lisgd
And lisgd can do any command you want when you for example swipe from left to right at bottom of the display.
I think there might be option to paste something so maybe you can even unlock the Phosh session with single swipe (I don't know how but it is most likely possible)
Touch screen calibration: no idea because I never had to do anything like it but weston should be the road with Phosh.
diederik: most linux mobile optimized apps are actually the same apps which you run on your pc or laptop. They are just developed to scale correctly and display menus etc. correctly also on smaller screens (and so it is easier to use them with touch screen).
For example with many apps (like gnome-settings) you can see sidebar when you are on large display -> if that app is optimized for mobile the sidebar is "hidden" or kind of in different tab because otherwise it would take 50% of the display. Also most of the navigation things are same but there might some difference where they are located on large display vs mobile (because by default you don't have mouse and keyboard with mobile)
Almost all developing work is done for Pinephone, Pinephone Pro or Librem 5 - none of them have physical back button.