Open Settings
Tap Display
Tap Screen timeout
Select the longest available time or Never, if available
On iPhone, go to Settings
Tap Display & Brightness
Tap Auto-Lock
Select Never
On Android, go to Settings
Tap Display
Tap Sleep or Screen timeout
Choose the longest available duration or Never
If Never is not available, enable Developer options if needed
Use a third-party app only if your device does not offer a no-timeout option
Keep the device plugged in if required by the setting or app
Check battery saver settings if the screen still turns off
Restart the device after changing the setting if needed