Post a screenshot of the Battery usage by app for the last 24 hours. Go to Settings > Battery, and scroll towards the bottom of the screen to reveal the apps along with the percentage.
Your phone getting hot for a couple of days after the update is normal while your files are being rendered, but after that it is due to the background processes on the apps that are running even when the apps are not opened.
After an update I recommend doing a Force Restart. It may take a couple of attempts to get the speed and sequence of the key presses correct and the last step of holding the side button until the Apple Logo appears could take up to 15 seconds. Ignore anything else on the screen during that time and keep holding until you see the Apple Logo.
Force restart iPhone - Apple Support