Are you getting a pop up message on your phone's screen saying it's “ overheating ” @S20ultra5gheating
The phone will warm up depending on what's being actioned on it in the background and what's been accessed by the user.
Some generic troubleshooting tips are …
1. Clear your phone's System Cache Files by turning your phone Off and then from a set series of button presses go into the phone's System Files and clear the cache. No information is lost this way. You may need to insert your headphones or hook upto a computer with your data cable to get to this menu on the phone.
2. Put the phone in Safe Mode which disables your downloaded app's but let's your stock app's keep working. If the issues stop then this highlights a downloaded app as the possible issue. Come out of Safe Mode and start deleting your most recent downloaded app's one at a time until it stops. https://www.samsung.com/uk/support/mobile-devices/how-to-start-my-galaxy-device-in-safe-mode/
3. Back up , remove any relevant sd card and factory reset.
4. A Samsung Experience Store or Samsung Service Centre or Samsung Doorstep Repair Service ( if applicable where you live and if the stores are open due to the pandemic ) can help.
I wish you all the best with this.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.