what's the name and function of this "screen swipe" in iOS?

When using my iPhone (with latest iOS 14.7.1, seeing it for a while...) I sometimes have the following behavior:

• the screen scrolls down by one third,
• the top one third of the screen is black, but I can still swipe the screen up (upward gray chevron)

this behavior is shown in this screenshot (while opening signal.app):

I have difficulties reproducing this behaviour or the sequences of gestures, still it happens quite frequently. I am very puzzled about its potential function - I am certainly missing something, and tried various combinations of gestures without any other possibility than just returning to initial state. any clue?

The behavior you are looking at is an iOS feature called Reachability. This feature was made available for large screen iPhone (4.7" and larger) to enable easy single handed usage.

Reachability lets users easily access out of reach, on-screen elements on their iPhone with single hand usage. It lets you temporarily bring items at the top of the screen down to the lower half of the screen. This way, when using your iPhone with single hand, you can tap on out of reach on-screen elements.

You can turn Reachability on or off by going to Settings app → Accessibility → Touch.

• On an iPhone with Face ID: Swipe down on the bottom edge of the screen.

  

• On an iPhone with a Home button: Lightly double-tap the Home button.

  

You can learn more about this feature here:

• https://support.apple.com/en-in/guide/iphone/iph77bcdd132/14.0/ios/14.0#iph145eba8e9

It's called Reachability. Its purpose is to make the top of the screen reachable with your thumb when you hold your phone at the bottom.

On an iPhone with home button, you can trigger it by lightly tapping the home button two times (not completely pressing it, just tap lightly).

You can switch this on or off in the system settings under Accessibility > Touch > Reachability.