It’s maddening to want to read a serial number or another bit of fine print that you can barely see. But fret no longer your iPhone or iPad makes a fabulous magnifying glass! Assuming Magnifier is enabled in Settings > General > Accessibility > Magnifier.
You can bring it up by pressing the Home button (for Touch ID devices) or side button (for Face ID devices) three times quickly. If that’s too hard to remember you can also add a Magnifier button to Control Center in Settings > Control Center > Customize Controls. The special camera viewfinder is zoomed automatically but you can change the zoom level with the slider, tap the flash icon to turn on the LED light (if available on your device) enables a filter to change the color or contrast or lock the focus by tapping the lock icon.
You can also freeze the image by tapping the white shutter button which is great for grabbing a picture of a tiny serial number on the back of some device (tap that button again to resume using Magnifier). To leave Magnifier, press the Home button or swipe up from the bottom of the screen.