The headline camera change is a variable aperture on the 48MP Fusion Main camera. Every iPhone until now has had a fixed opening — the phone controls exposure with shutter speed and ISO alone. The 18 Pro can physically open and close the aperture, which does two useful things.
Wide open, it pulls in more light, so low-light shots need less computational rescuing and hold onto more real detail. Stopped down, it deepens the field of focus, so a group photo or a landscape stays sharp from front to back instead of quietly blurring whatever wasn't the focus point. Compact cameras have done this for decades; getting it into a phone body this thin is the achievement.
Apple has paired the new sensor with new Pro controls and Photographic Styles 3, which add finer adjustment over texture and grain after the shot is taken — more creative range for anyone shooting RAW or editing on the phone itself.