You also need to be pretty precise with attaching the phone in an exact position, and grinding things with a very low margin of error, say, 0.05 mm along the vertical axis, in order to remove the chip but leave the PCB intact. Not for the faint of the heart.
I suppose they have had several training rounds using badly broken phones first.
Yrs, as long as the milling machine is adequate. Hobby-level CNC devices may have harder time doing that, and pro / industrial devices cost many times the iPhone.
Yes, the flash chips are approximately the same in their mechanical properties. Milling speed, debris removal, depth of milling to keep the PCB intact, treatment of the PCB.afterwards to clear it and attach another chip. It all is best done several times on dead bodies before attempting a modification of the expensive target phone.
I suppose they have had several training rounds using badly broken phones first.