How can Koreans stay in China for a long time?

From 65438 to 0992, after the establishment of diplomatic relations between China and South Korea, many Koreans were sent to work in the metropolis of China. With the rapid development of China, many companies went bankrupt. Many Koreans living in metropolises gradually moved to various parts of China, and many Koreans returned to their own countries. Among them, because their children grew up in China, many people have been living in China and have not returned to China for fear that their children will be out of breath in South Korea. Now, those Koreans who have stayed in China for more than 20 years don't want to go back to China because Korea has become too big and there is nothing to do, so they have been living here until today. In this way, some families stayed in China for a long time.