iPhone 15 keeps restarting randomly

First thing to check for random restarts is the battery.

Settings > Battery > Battery Health & Charging

What is the current Maximum Capacity?

How many Charge Cycles has the battery completed at this point?

Are you the original owner? About how old is the iPhone 15?

it's unclear what the problem is, it could be anything. But performing a hard reset could help this problem. Here's how:

Perform a True "Force Restart"

A standard power-down doesn't clear the deep hardware cache. If a rogue system process is stuck in a loop, a forced hard reset can snap it out of it.

1. Press and quickly release Volume Up.
2. Press and quickly release Volume Down.
3. Immediately press and hold the Side Power Button. Keep holding it down through the black screen, and let go when you see the Apple logo appear.

If this doesn't work, contact apple support here: Apple Support