"...Auto Night Sight: In standard Photo or Portrait mode, the camera will detect low-light scenes and turn on Night Sight automatically. You do not need to manually switch modes.
Manual Flash: To use the physical flash, tap the Settings icon (gear/arrow) in the bottom-left corner of the camera app and select On. Note that using the flash disables the automatic low-light photo processing.
Forcing Night Sight: To manually force the feature without waiting for auto-detection, swipe the mode carousel to the dedicated Night Sight option..."
Most of the time you don't need a flash. That's part of the point of the pixel the software can get a photo in the dark without a flash. The auto flash basically always on. But it's almost never dark enough to trigger it.