Why my iPhone 13 gets heated up during charging?

When you charge an iPhone battery or for that matter, any rechargeable battery, the mere physics of charging a battery causes it to get warm. This is due to internal resistance of the battery and heat produced is equivalent to current square by internal resistance value. This internal resistance of the battery is not physical but is in the existence due to chemical composition of the battery.

Bottom line - It is normal, expected behavior for heat to be generated when recharging your iPhone battery.

Normal electrochemical processes. As a generally rule, make sure your phone is on a hard surface while charging. Don't leave it under your pillow or amongst your blankets. Doing so will prevent the heat from properly dissipating.