One group is the Joseon people. The Joseon people are believed to have originated in Manchuria, and they migrated to the Korean peninsula around 2,000 BC. They established the first Korean state, Gojoseon, in the 4th century BC.
Another group is the Silla people. The Silla people are believed to have originated in the southern part of the Korean peninsula. They established the Silla kingdom in the 6th century AD, which eventually unified all of Korea under its rule.
Finally, there is the Baekje people. The Baekje people are believed to have originated in the southwestern part of the Korean peninsula. They established the Baekje kingdom in the 4th century AD, which was one of the three rival kingdoms of Korea along with Silla and Goguryeo.
These three groups are considered to be the indigenous peoples of South Korea. However, there have been many other migrations and invasions throughout history, and the Korean people are a mix of many different ethnicities.