Not all Koreans have smooth skin. Like me.. xD But I guess a lot of Korean people have smooth and flawless skin.
Judging from watching my mom and grandma, Koreans use a lot of creams and moisterizers. They're different creams and moisterizers for different benefits, like maybe wrinkle diminishing or whitening creams.
But obviously, they do use various skincare washes and moisterizers daily so that they're skin can become flawless. I also think that diet may have to do with Korean people's skin. I have noticed by observing my baby cousins and relatives skin that ever since birth, Koreans have very soft and radiant skin. Although, like all ethnicities, Koreans get pimples and large pores. But in general, many Koreans have very smooth and soft skin despite having pimples and large pores. The general surface of Koreans skin is very smooth and soft.
However, like an other person answered, Koreans are obsessed with plastic surgery. Many celebrities that you see on T.V. have received surgery and therefore have fixed and flawless "plastic" skin. I'm not entirely sure, but perhaps as Korean celebrities, receiving laser surgeries to erase pimples and scars is "required". And that is how they reach they're almost too perfect skin.
I'm not expert, but judging on my experience and thoughts, that is what I believe.
Hope that helped!