Updated: Nov 29, 2019
It’s call “screen recording”
Why learn this? You might want to teach someone how to do something on your iphone and then send them the video in the form of picture video file.
If you've ever sought to download an app, you would have seen a moving video of how the app works. This may have been done using this "Screen Capture" on your Iphone.
PS These hacks work on iphones that are quite old eg my Iphone 6.
Those who have android phones (or other operating systems), ask Dr Google for the equivalent instructions relevant to your phone type.