The double tap feature on your iPhone lets you take a screenshot just by double-tapping the Apple icon on the back of your phone, and YES, it works with a case on.
Since all of these videos seem to be on Tik Tok, which is really frustrating if you don't have the app, I made a quick VIDEO BELOW to show you how to change the settings to use the double (or triple) tap feature on your iPhone.
Follow these simple instructions:
Settings >> Accessibility >> Touch >> Back Tap >> Choose double tap or triple tap >> select 'Screenshot'
Then you can double or triple tap the Apple icon on the back of your phone to take a screenshot!