One way qin shihuangdi United China was by

Qin Shi Huang was the first emperor of a unified China as he made the first dynasty of China.

Explanation:

Qin Shi became the ruler of the Qin region as a young boy of 13 and when he came of age he began to expand his military might.

First he took over his neighboring states by force and then used various diplomatic and opportunistic ways to take over the rest of the warring states of China while establishing the aristocracies there as well guarded in their status.

This unified China as a country with the warring states placated under one strong ruler.