No preliminary analysis has made it clear where the original Japanese people came from, so you have to be content with the limited information on how Japan began to work as a country. Japan started as a country in the 7th century.
Before that, the island that would be Japan was like the USA in the first half of the seventeenth century. I mean the immigrants came there from different countries and regions, including South Asia and Polynesia. Of course, before they came there, some peoples must have been there, like Eskimos.
From the 4th or 5th century, a powerful people from the Korean peninsula moved to Japan, who tried to survive on tree in Japan because they need them to make iron. Some researchers say that they were so aggressive that they were described as a big snake with eight heads in a myth. Anyway, those Koreans were one of the major peoples in ancient Japan.
As to the dawn of Japan, one of the biggest mysteries was why so many small communities in ancient Japan were integrated into one country without any conflicts. As shown above, there were several peoples, each of them had a community, so it seemed to be difficult for them to have a whole country without any competition, but they did it peacefully.
They needed to be united into a country in the beginning of the 7th century because they were afraid China would assault Ancient Japan consisting of small communities. In fact, China and Kogryo attacked Paekche in the south of the Korean peninsula, and Japan went to help Paekche, but lost the war. Their threat required small communities in Japan to be a whole country.
