My iPhone 11 keeps restarting by it's own even after updating it

you may have a hardware problem

First try a forced restart

https://support.apple.com/en-gb/guide/iphone/iph8903c3ee6/ios

If that does not help the  next procedure would be to backup then restore your device to factory, and setup the device without restoring a backup, to see if the issue is replicated. 

Then restore the backup.

Here is a guide for those steps:

Backup and restore

(1) How to back up your iPhone, iPad, and iPod touch – Apple Support

(2) Restore your iPhone, iPad, or iPod to factory settings – Apple Support

(3) Restore your iPhone, iPad, or iPod touch from a backup

If that fails arrange a repair

https://support.apple.com/en-gb/iphone/repair/service