Iphone randomly turning off and back on

Try a Forced Restart on the phone. Follow the instructions very carefully, as this is a bit tricky. (Hint.....ignore the "slider" when it appears and keep holding the Side button in until the Apple logo appears. This might take another 30 seconds.)

Force restart iPhone - Apple Support

There are only 2 possible results when you perform a Forced Restart:

1) The phone returns to normal operation. Otherwise, if the phone is still restarting at random......

2) There is a hardware problem with the phone that will need attention at the Apple Store (make an appointment) or take the phone to an authorized Apple Service shop.

You can find a list of authorized service shops in your area using the link just below:

Find Locations