You can use your i-phone to set your screen continuously on by going to the settings- Display and Brightness- Auto- Lock and then choose none.
This does not allow the automatic turn-off of the screen.
Remember that it can consume more battery thus turn it off when it is not necessary.
To keep an iPhone screen on, go to Settings > Display & Brightness > Auto-Lock and select Never. This prevents the screen from turning off automatically. Keep in mind it may drain battery faster, so use it when needed and switch back later.
