Phone cameras can indeed be hacked by someone who knows what they're doing but it's very unlikely that your phone camera would be hacked by anyone.
The hacker can install spyware on your cellphone which they can then use to control your cellphone and camera even when the phone screen is off.
If the hacker gets into your phone through spyware the hacker will be able to view your pictures and take photos as well as videos using your camera, even when its screen is not active.
If you think your camera or phone has been hacked you should take it to the phone company and let them know about it as they should be able to see if there is spyware on it and fix the problem.