SEG Plaza, one of the tallest skyscrapers in Shenzhen, China, has about 70 stories and thousands of tenants. During the lunch rush, the wait for an elevator can stretch to half an hour — a nightmare for a food-delivery driver trying to fill as many orders as possible.

The solution? People like 16-year-old Li Linxing.

As lunchtime approaches on a Monday in August, he positions himself near the building’s entrance, eyes scanning the surrounding streets. When a delivery driver appears, Linxing thrust his hand into the air and shouts: “Delivery stand-in!”

The driver, still straddling an electric scooter, hands an insulated takeout bag to Linxing, tells him which floor it goes to and scans the QR code that Linxing wears printed on a card hanging around his neck, to pay him 2 Chinese yuan, about 28 cents. The whole operation takes mere seconds. And then the driver speeds off to the next destination.

The last leg of the journey is up to Linxing. He squeezes into the elevator lobby with dozens of other people — many of them fellow delivery runners — to go up to wherever his hungry office worker awaits. Then it is back downstairs and repeat for the rest of the day, from lunchtime through the afternoon and into dinner, until around 8 p.m.

This logistical feat, a gig economy within a gig economy, is another example of the entrepreneurial spirit of Shenzhen, a city of about 18 million in southern China that pioneered the country’s embrace of a market economy. None of the dozens of last-mile delivery workers who mill outside SEG Plaza every day has a formal contract. Most are retirees, although during the summer, teenagers like Linxing appear, too. Some are there out of financial necessity, others for fun or exercise.

All of them are there because there was a need, and someone had to meet it.