> Powering something off of any old USB port means it would have to fit within the 5V 500mA basic specification, which the Raspberry Pi 3 exceeded long ago.
Theoretically, devices like the iPhone have lower power consumption than that and loads of performance features like recording 4k 120fps video.
Of course, an iPhone costs much more than a RPi, and has much better economies of scale, so they’re not truly comparable.
Your iphone (or other such device) can't do those things on basic USB charging alone, it'll be topping up the power budget considerably from the battery.
Theoretically, devices like the iPhone have lower power consumption than that and loads of performance features like recording 4k 120fps video.
Of course, an iPhone costs much more than a RPi, and has much better economies of scale, so they’re not truly comparable.