iphone gets very hot when charging but isnt as hot when charging and using

When you’re just charging, your iPhone is fast charging, which produces more heat. When using and charging, your iPhone disables fast charging automatically.

iPhones use passive cooling, meaning they don’t have a fan or other device cooling them. Passive cooling dissipates heat several ways, convection and conduction.

Conduction occurs when a heated surface comes in contact with a cooler surface. This is what happens when you lay your iPhone down on a surface while charging. Heat transfers to the cooler surface, thus cooling the iPhone. However, many surface materials do not conduct heat well. Some surfaces are actually insulators and make your iPhone warmer. Soft surfaces like pillows, blankets, soft materials are insulators.

Convection happens when the back of your iPhone is exposed to air. This is typically when you’re holding your iPhone using it. The air next to the back surface is heated and rises. The rising air is replaced by cooler air creating a constant source of cool air to cool the iPhone.

Are you seeing an "Overheating" message appear on the screen? If not, the phone is not overheating. It won't hurt to use the phone while the battery is charging, if you want to do this.