The entire back of your iPhone acts as a secret "button" (and yes, it still works with a case). You Can Open Any App By Tapping the Back of Your iPhone: Here's How - CNET X ...

Open the main Settings app from your iPhone home screen and tap Accessibility. In the Physical and Motor section, tap Touch. Scroll all the way to the bottom and select Back Tap.

Back Tap is an Accessibility feature in iOS 16 — first introduced back iOS 14 — that allows you to map commands or features to "taps" on the rear of an iPhone.

Note that Back Tap requires an ‌iPhone‌ 8 or newer model running iOS 14 or later. Launch the Settings app on your ‌iPhone‌. Tap Accessibility. Under "Physical and Motor," tap Touch.