In a statement, the Zhengzhou factory’s owner, the Taiwanese electronics supplier Foxconn, declined to comment directly on the group’s figures but said it was constantly reviewing its operations and planned to add more than 50,000 positions across China in the first three months of the year.

An Apple spokeswoman, Wei Gu, pointed to the company’s previous statements and declined to comment further.

Huojiancun is a rough industrial community on the edge of glittering Shanghai. Snack shops, calling card stalls and other small businesses catering to factory workers pervade the narrow streets and alleys surrounding the Changshuo plant.

Workers there may make the iPhone, but they can’t afford it. Even when their pay was at its peak, those assembly line workers would have needed more than a month’s wages to buy a base-level iPhone XR. Still, jobs there for years were in high demand because they tended to pay more and offer better working conditions than elsewhere.

Today, the situation is more complicated. Chinese workers are increasingly difficult to lure because they don’t want to work so many hours on an assembly line. Chinese companies are increasingly exploring automation as labor costs rise.

Supervisors kept workers from taking too many breaks and would dock their pay for infractions like littering, said Hou Fu’an, a Changshuo worker who had quit and was heading back home in the interior province of Henan. As workers began to leave over missing hours, he said, many remaining workers started getting overtime again, fattening their paychecks but adding to the workload.

“Inside the factory, workers are tightly controlled,” Mr. Hou said. “If one person does not do well, everyone in the group will be yelled at by the group leader. But they did not yell much when there were not many workers left.”