The attached article and other related links may give you a basic but clear understanding of the origin of Korea. As you know, today, the arrogant (south) Koreans are liable to believe the fake history they have fabricated by themselves; pathetically, most of them do not feel shameful about it.   

一篇文章弄懂讓人傻傻分不清的「高句麗」和「高麗」 - 每日頭條 (kknews.cc)     

PS. I remember I told you before that in college, a classmate from South Korea argued with us about a silly topic: who are prettier, Chinese women or Korean women? Out of politeness and praise, we all said Korean women are prettier. But he insisted that Chinese women were prettier because, in the conquest of the Tang Dynasty over Goguryeo, the Tang removed all of its royal families, including all beauties, to Tang territory, since there had been no pretty women on the peninsula any longer. He was partly right; nevertheless, he still regarded the Goguryea as the ancestors of modern Korea.