This article explains how to use Picture in Picture mode on an iPhone running iOS 14 and later.
How to Enable Picture in Picture Mode on iPhone (Turn on PiP)
Picture in Picture (PiP) mode on iPhones makes multitasking easier. For example, you can watch a video in a supported app like Vimeo or make a FaceTime call while doing something else.
During a FaceTime call, press the Home button or swipe up to view the Home screen.
The FaceTime call window will shrink to a thumbnail. You can now switch to another app during the call. Return to a full FaceTime screen by tapping the small maximize icon.