It's obvious. As soon as you touch the flashlight button, there is haptic feedback along with the button growing, exactly like the camera button on the other side. And yes, the button grows beyond the size of a 95th percentile finger. Now, whether or not you pay attention to the physical and visual cues is up to you. The cues are there. The instructions are in the official online guide as well as "About 3,020,000" other pages. They are in the official user guide in the books app. It's 'discoverable' without much of cognitive load for anyone that has used a phone in the last 20 years, not 'immediately obvious'.