According to Engadget, the iPhone's Always-On Display is available only on certain recent models: the Pro models from the iPhone 14 through the iPhone 18, plus the standard iPhone 17 and iPhone Air. The feature is on by default, and users can check it under Settings > Display & Brightness > Always On Display.
Some shutdowns are intentional. The display turns off when the phone is face-down, blocked from view, away from a paired Apple Watch, or unused for a long period. It also disables during CarPlay sessions and Continuity Camera use, and those two cases cannot be changed by the user.
The adjustable causes are Low Power Mode and Focus modes. Turning off Low Power Mode, or disabling Adaptive Power on iPhone 15 and later, prevents the phone from re-enabling it automatically. Sleep Focus can also force the display off; users can turn off its schedule in the Health app or add an Always On Display Focus Filter. The guide also suggests toggling Raise to Wake and restarting the iPhone if the problem continues.