Fix for iPhone turning off on itself and not turning back on
How to stop your iPhone shutting down unexpectedly
How to Turn Off Your iPhone 12 Pro Max: 2 Easy Ways
Stop iPhone from Shutting Off Randomly - 6 Tips
Why is the iPhone battery health 90% but still the phone randomly shuts down at 86% charge? It doesn't charge beyond 84-85%. The device is an iPhone 12 Pro. - Quora
How to Power Off, Power On & Hard Reset iPhone X & Later | www.iphonelife.com