(id like to preface this by saying im not korean and i live in america) i wouldn't say 90% of koreans have clear skin, i have seen quite a lot of koreans at my school with varying levels of acne. part of this perception could be due to the fact that skin care is a huge craze in korea, with some kind of skin care store present on every corner in seoul similar to how mcdonald's and starbucks is everywhere in america.
also keep in mind that facial appearance is something of great importance in korean society and people will do more than average just to upkeep a "perfect appearance" (heavy picture filters and angles, makeup, medical supplements)
everyone's skin is different and some remedy it with dilligent skin care routine, other people's acne is hormonal and have to take things like accutane (for some women, taking birth control helps with hormonal acne but can make it worse for others)
personally my acne improved greatly by washing my face everyday with ivory soap and moisturizing right after so that my skin doesnt overproduce its natural oils, which i believe is what caused my acne (on top of the dirt that accumulates over the course of the day), luckily i didn't have to go on accutane like one of my friends had to do, which affected his liver while he was on it