If you put up the monitor, Foxy automatically fails any movements he tries to do and will start his countdown to moving again from a few seconds to never, but don't try it.

Same for Freddy, but he works differently. He has a countdown to move to the next room, putting up the monitor only prevents him from moving, keeping his countdown going. If his countdown goes to 0 and the monitor is up, he will only move as soon as you put the monitor down.

But at the wallway corner camera, he works differently. As soon as you put your camera to one other than the east hall corner (where he is) and the door is open, he gets in. Also, when jumpscaring, he doesn't put down the monitor himself. So if he's in, flip the monitor and check the doors to be safe, but be extremely quick.