From The Apple Wiki

System PiP (Picture-in-Picture) allows content to be displayed in a floating window on iOS/iPadOS. On iOS, PiP for video playback is available on iOS 14, whereas video calling via PiP requires iOS 15 or later.

In prior versions of iPadOS, Apple would sometimes grant special privileges to large developers (such as Zoom[1]) to allow their calling apps to work via split screen view, rather than PiP.

Non-system PiP

Some apps, such as Signal and Telegram, do not support system PiP but instead present a tiny 'window' inside of their main app view. This PiP 'window' does not float outside of the main app's space, therefore it cannot be used for multitasking--switching to another app pauses the user's camera so that the other party cannot see the user.

Apple documentation