Usually when your phone is left on charge and it hits 100%, your phone still receives electronic signals from your charger and is then hit with a loop of going to your phone, being rejected and sent back out again by your charger. This will cause your phone to get very hot and is a risk of fire. I suggest that you ensure your phone is unplugged when it hits 100%.
Another cause may be due to applications being open, your phone will overheat if too many applications are trying to run at once, you can fix this by simply closing applications after you've used them. You can also manually stop applications from running in the background via settings. Kind of like your brain, too much work and it begins to stress. This is just your phones way of letting you know it's stressed.
sorry but it does not work like this 😂 y9u can leave you phone on for as long as you want on charge when the cable is connected and its at 100% the phone will actually start to discharge a couple of % so for instance it will go from 100% down to 98% then charge its self back to 100% this is a standard feature on pretty much all phones to stop fires etc