South Korea's C38 visa to work in South Korea has expired for three years, and it is always two or three months to return to China once a week or so. Can I still apply for c.38?

You can apply for C38 directly. Now this c38 logo is valid for 5 years. This is a tourist visa. It is best to participate in the technical education lottery of H2 visa. H2 visa (3 years) is a work visa, so you don't have to come back every two or three months. There is also a skill qualification exam. You can change F4 visa (valid for 5 years) after passing the exam, which is also a legal job.