India, Sept. 1 -- During his first year at Apple in 2001, John Patrick Ternus found himself at a supplier facility late one night, far from home, with a magnifying glass for company. He was counting grooves on the heads of the screws that were to be packaged with the Apple Cinema HD Display that would eventually launch in early 2022. There were 35 grooves chiseled. Apple's specification called for 25. Ternus argued. Perhaps needless to point out, he won. The supplier changed the screw design to have exactly 25 concentric grooves.

On September 1, John Ternus, 51, is no longer only a person who builds Apple's products, but also becomes the person responsible for all of them. After 25 years inside the company, the last several spent running...