Your iPhone screen will remain on if you are interacting with the screen, but as soon as you leave the screen idle, you seen will go off into Auto-Lock in 30 seconds (ideal default setting or when in Power Saver Mode)
If you have a use-case where you want to keep your iPhone screen on for long you can do that by the below steps,
- Go to Settings Application,
- Go to Display & Brightness,
- Tap on Auto-Lock,
- Now you can select Auto-Lock time from the below options available,
- 30 seconds
- 1 minute
- 2 minutes
- 3 minutes
- 4 minutes
- Come out of Settings, now your screen will remain lit until the time you have set for auto lock.
✌️If you have Low Power Mode turned on, you will see that the Auto-Lock is defaulted to 30 seconds and disabled i.e. you will not be allowed to increase or decrease auto-lock settings.