She’ll dip her hands into a tray of water, to determine whether the temperature is just right. She can explain the intricacies of heating glass in a potassium ion bath. When she passes a grinding machine, she is apt to ask technicians to step aside so she can take their place for a while. ... Ms. Zhou knows the drill. For years, she labored in a factory, the best job she could get having grown up in an impoverished village in central China. ... Ms. Zhou has honed her hands-on knowledge into a world-class, multibillion-dollar operation, one at the vanguard of China’s push into high-end manufacturing. Lens Technology is now one of the leading suppliers of the so-called cover glass used in laptops, tablets and mobile devices, including the Apple iPhone and the Samsung Galaxy. This year, her factories are expected to churn out more than a billion glass screens, each refined to a fraction of a millimeter. ... In 2003, she was still making glass for watches when she received an unexpected phone call from executives at Motorola. They asked if she was willing to help them develop a glass screen for their new device, the Razr V3.