Did the Japanese separate during the Qin Dynasty?

No, but some Japanese are of China origin. This is because Xu Fudong crossed Japan.

In 2 19 BC (the 28th year of Qin Shihuang), Qin Shihuang made his second tour, and a large group of people sealed the temple in Mount Tai and carved stones, then went to Bohai Sea in a mighty way. Arriving at the seaside, Qin Shihuang boarded Zhifu Island and enjoyed himself. I saw people in the mountains and rivers between the sea of clouds, which was spectacular, especially fascinated Qin Shihuang. This scene was originally a mirage, but the alchemist described it as a legendary fairyland on the sea in order to cater to Qin Shihuang's desire for longevity. Chui fook took the opportunity to write to Qin Shihuang, saying that there are three Fairy Mountain in the sea, Penglai, Abbot and Yingzhou, where immortals live and can get immortal medicine. Qin Shihuang was very happy, and then, according to Xu Fu's request, he sent thousands of men and virgins to the sea with him to get the fairy medicine. He himself lingers here, waiting for Xu's good news. However, only Xu Fu returned empty-handed. Xu Fusheng said that he wanted to see Poseidon, who refused to give the fairy medicine on the grounds that the gift was too thin. In this regard, Qin Shihuang was convinced that he sent 3,000 boys and girls, craftsmen, technicians and grain seeds to make Xu Fu go to sea again. Qin Shihuang refused to leave, waited for three months, and didn't hear from Xu Fu, only to be disappointed.