In the southern Chinese city of Shenzhen, a mazelike market stretches for a half-mile (805 meters), packed with stalls selling every type of electronic imaginable.

It’s an open secret that vendors here are offering one of the world’s most sought-after technologies: the microchips that create artificial intelligence, which the United States is battling to keep out of Chinese hands.

One vendor said he could order the chips for delivery in two weeks.