Please check in your phones settings that any auto restart hasn't been toggled On.
It might be worth just checking in your Playstore to see if there are any pending updates for your apps.
If there isn't any then perhaps put the phone in Safe Mode which disables any downloaded apps and leaves the stock ones working and then see what happens.
If the issue stops then one or more of your apps are probably causing this problem.
Come out of Safe Mode and start deleting your most recent apps one at a time until it stops.
If the issue continues in Safe Mode then I'd suggest to turn the phone Off and then from a set series of button presses go into the phones system files and clear the cache.
If this does not help then back up and then remove any relevant Sd card and factory reset as a last resort.
If that does not help then let a Samsung Experience Store or Samsung Service Centre have hands on @Johnnybob72
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.
I am also facing the issue . The service centre guys are suspecting a hardware issue , but I am sure it is due to the software issue . Many people have reported the issue in community. I cant just throw away my phone after 1 year use. The fact that this happens right after my warranty expired date is making me suspicious