It's when you swipe up from bottom of screen on Apple (and Android if you've set it up to use gestures), or when you tap the App Overview button on default Android (App Overview is usually the leftmost of three navigation buttons at the bottom of the screen).

I didn't know this popup view existed, but here's how to trigger it: https://www.t-mobile.com/support/tutorials/device/samsung/galaxy-s10-plus/topic/settings/how-to-run-apps-in-a-pop-up-view

For what it's worth, I can't get copying text to work in it on Thunder.