Why does my iPhone keep rebooting?

See if the things in this article help: If your iOS or iPadOS device restarts or displays the Apple logo or a spinning gear unexpectedly - Apple Support

If not, contact Apple: https://getsupport.apple.com/

Two basic troubleshooting steps to perform (test after each attempt):

A) Try a "forced" restart of your device: Put it on charge for at least half an hour, then, keeping it plugged in, use these instructions: gestures:

Press and hold at the same time the Power button and the Home buttons. Only when the Apple logo appears (it might take 10-20 seconds), release the buttons

B) If you can, check your battery status in Settings>Battery>Battery Health>Maximum capacity. If it is close to 83-80%, have it replaced as soon as possible, since it might have unpredictable behavior.

If the issue still persists, it might be a hardware (battery? digitizer? other internal component?) failure, and I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region . After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.