Why does my iPhone 16 Pro Max turns off automatically

1. Make sure you have the latest updates installed.
   1. Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
   2. Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)
2. Follow the Apple article to resolve the issue --> If your iPhone, iPad, or iPod touch won't turn on or is frozen ...
3. If not resolved; please follow the steps in the following link for support with this issue: If your iOS device restarts or displays the Apple logo or a spinning gear unexpectedly

You may have Auto-Lock turned on. Go to Settings > Display and Brightness > Auto-Lock > Set it to Never. While this will make your iPhone never turn off, this opens a security vulnerability for anyone picking up your iPhone and being able to access all of your personal data.