It's from today's Apple event, and the scene is a young couple moving into a house together. The girlfriend video calls another man and starts chatting, and the boyfriend is left looking at a video of this other man on the phone's second display.

The joke is not "it looks like the girlfriend is cheating on him", but rather "showing a video of the person you're calling on the display facing away from you has no obvious use cases except for cuck play"