Have the S10+ 5m lb and noticed it has restered several times. Once just now with 20% battery and moderate use, once last night charging and at least one other occasion. None of the restarts were due to software updates.
The phone can be set to reset itself via Settings > General Management > Reset > Auto Restart.
If this is not toggled On then please put your phone in Safe Mode which disables your downloaded apps and leaves the stock ones working which can help to determine if this is a rogue app causing this.
For example CleanMaster app has been known to cause conflicts.
If it continues in Safe Mode then I'd suggest to get some support from a Samsung Experience Store or Samsung Service Centre to have a diagnostic test on your phone.
Depending on what Cooling Off period you have I'd also consider asking the supplier of the phone for a new one.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.
I found the weekly reset was changing the date it was to run. Appears that it was incrementing the day and resetting every night. Turned that feature off, and no more resets