There are a few troubleshooting steps to take here the first would be to try running your Note 9 in safe mode and see if the problem still occurs.
To do this,
Assuming the problem doesn't occur in safe mode the problem could be caused by an app. Usually an unofficial third-party app. I suggest you go through each one and remove them one by one until you find the culprit.
If the problem still occurs it could be a software issue and your device may need to be factory reset.
Be aware you will encounter data loss at this point.
To proceed to follow the link below.
If all else fails it could be an issue with the hardware, more than likely your motherboard. In that case, I would suggest you call 1-800-SAMSUNG to further escalate this situation.
I hope this was helpful!
It's possible it could be a software or hardware issue but the easiest way to tell is to back up your device up on Samsung smart switch via memory card and doing a factory reset. Once you go to restore your data restore everything but your applications. This process will do 2 things for you, it will clear out possible corrupted software and will determine if you have an app causing the restarts. Use phone for 48 hours with out the apps. If the phone doesn't restart again great then you can restore your apps via smart switch. If this continues after the app restore that means you have an application causing the phone to reboot. If the phone reboots within the 48 hours call 1800 Samsung for a service request. Before you factory reset your device please make sure you either know or remove the google account on the device or you can be locked out.