Open Settings
Tap Display & Brightness
Tap Auto-Lock
Select Never
If Never is not available, check Low Power Mode and turn it off
Go to Settings
Tap Battery
Turn off Low Power Mode
Keep the iPhone plugged in if needed for longer screen-on time
Use Guided Access if you want the screen to stay on during a specific app
Go to Settings
Tap Accessibility
Tap Guided Access
Turn on Guided Access
Open the app you want
Triple-click the side button to start Guided Access
If the screen still turns off, check for Screen Time or device management restrictions