Car Overheats When AC Is On - Here is Why! 

This is actually explained really well.

There is one other potential cause not mentioned that I experienced,. Grill shutters are fitted in front of the radiator of many if not all conventional engines. They are on both my 2020 F150 and my 2012 Focus on which I was having an overheating issue. I also have seen them on dodge and Chevy trucks. They are said to increase fuel efficiency and speed the engine's reaching running temperature, but when the AC is on all that is overridden and they stay open as the engine may need the excess cooling capacity the added air flow offers. If they become inoperative and stuck closed the vehicle can overheat in traffic on hot days.