How did China move from the Warring States period to Shi Huangdi's successful control of the entire

Many small and medium-sized vassal states have been gobbled up, and the rest of the Qin, Chu, Yan, Han, Zhao, Wei, Qi - seven countries become the main vassal states during the warring period. Shang Yang’s reform played an important role in Qin, and Qin got strong, defeated the other six countries and unified China.

It was the last of three periods of the Zhou Dynasty era (1046–221 BC): Western Zhou Dynasty, Spring and Autumn Period (771–476 BC), Warring States Period
States declared independence from the Zhou Dynasty, and kingdoms fought for territory, during this period.
By 334 BC there were 7 warring states: Qin, Chu, Zhao, Wei, Han, Yan, and Qi.
The Warring States Period ended with Qin's conquest of the other states.
It was succeeded by the Qin Dynasty in 221 BC, which marked the transition between ancient China and Imperial China.