Looking at the iFixit battery replacement guide for your device, [guide|98167] , I can see that the battery is located primarily on the left hand side of the device behind the screen and the logic board is on the right side.
The heat is most likely coming from the battery. Batteries do get warm while charging, even warmer when charging quickly, and can get hot or very hot as they get old. Typically a device will use power from the charger to run while charging the battery but not always. If it uses the battery to run the device while charging, this creates a lot of load on the battery and you should probably put it down or turn it off while charging. Something I’ve heard of happening moreso with wireless charging technology and drastically reduces the life expectancy of the battery.
Are you using the charger and cable that came with the device? This is highly recommended. Aftermarket or cheap chargers can do permanent damage to your device. Also, it may help performance as well to take steps in reducing battery usage on the Lenovo such as lowering screen brightness to a comfortable level.