I've been looking into the origins of Japan, and all I've came to read were the usuals of how people have migrated by foot to the archipelago, and later, how the 'landbridge' disappeared underwater.
So I remember back in gradeschool, during Chinese history class, being taught about Qinshi Huang and his borderline obsessiveness on finding eternal life: to the point of sending people out at sea for the so called 'fountain of youth'.
I'm not certain if this was what the teacher had taught me or it was just a theory I had as a child, but is it possible that these people who have been sent out by the first emperor of China, be the first people of the countries we now know as Japan and Korea?
Could it be that they brought their families with them in the ship, with no intentions to come back and fully knowing that if they come back empty-handed, they'd be beheaded.
I couldn't find any articles confirming this. And it's literally grating my mind. I might just not be looking hard enough though.
In addition, I'd just like to point out how some of their cultures are similar. I love the linguistic similarities and as a mandarin speaker, when I hear some Nihongo and sometimes look at kanji, it makes me think it won't be too difficult for me to learn it.
You might want to read on the Japanese Jomon period (14,000 B.C.), and on the Korean Jeulmun period (8,000 B.C.) to know about both regions probable first peoples.
The major historical sources of Korea are Samguk Sagi and Samguk Yusa which were written over a millennium after the purported founding of the Korean nation.
The mythical founding of the first Gojoseon goes to Dangun and the first written account of it is in Samguk Yusa. The text focuses on myths and legends of Korea, and is an important source but not necessarily a historical or factual one. Regardless, the first Gojoseon is assumed to be somewhere in Manchuria.
The successor state to this legend was a Chinese-Korean state found by 기자 (Gija in Korean, Jizi in Chinese), which is also a semi-legendary account found in Samguk Sagi. But this herein lies the problem of ancient ancient history. Current terms and national boundaries being applied to ancient states makes it difficult to evaluate whether they were Korean or Chinese or if they were either at all. Being outside the Chinese heartlands meant that they were considered barbarians (if after a few generations and mixing of peoples) while Korean identity hadn't been consolidated yet.
Finally, there's Wiman Joseon which was again was founded by a Chinese - but his records are also found in Chinese records of the same time period as well which makes it more reliable. But this is during the Han dynasty after Qin, and the continued colonization by the Han dynasty with their commanderies in Northern parts of the Korean peninsula is what makes ancient history so contentious to this day.
The proto kingdoms of Korea which include the mentioned above plus the tribes in the southern peninsula makes it more likely to say what we considered as Chinese and Koreans and Japanese today mixed and borders were far more fluid. Consolidation of national identity is an anachronism that we apply to make it I guess, easier to comprehend what is and isn't ours but such notions didn't exist in ancient history.
As far as Qin Shi Huangdi's expedition goes, I think there's a blend of legend and reality that likely points more towards a mix. Korea in its consolidation after Silla and founding of Balhae in the northern portion where Goguryeo existed is a mix of Jurcheon and Koreans, Chinese and Koreans, Japanese and Koreans, etc. Furthermore, during the three kingdom period, the fall of Baekje led to immigration of the nobility and some peoples to the Japanese islands. All this points to fluidity in ethnicity, hardly isolation even as history entered Goryeo and Joseon periods. But movement of peoples in East Asia happened much earlier than the Qin era.
My family keeps a record of members of our clan 본관 (I'm Korean), and I've heard from my grandparents and uncles that our family were originally immigrants from China. I do not know if this is true, but I don't doubt it either. Plus, many other clans in Korea are from China as well. I don't take this to mean that we are Chinese however.