Ive got the converters to plug my ps4 into your fridge, but howd he change the input on the screen?

He had access to the back of the monitor. It was just mounted on a frame. He probably unplugged the input and plugged into the same port.

https://www.wxii12.com/article/passenger-takes-over-airport-monitor-plays-video-game-while-waiting-for-flight/30573509