It was Sunday when Dong Wanwan decided to give up her job at the world’s largest iPhone factory and walk home. The 20-year-old had been working for the past three months on the production lines at Foxconn Technology Group’s plant in Zhengzhou, one of tens of thousands helping put together Apple Inc. iPhones that would be sent around the globe. It was a coveted job, among the country’s best-paid blue-collar gigs. Then Covid began spreading on the manufacturing campus. The factory went into a “closed loop,” walling off the giant complex from the outside world. Trash piled up in hallways....