A lot of Korean texting slang has a rooted meaning in the original word, just shortened to make it easier to type. Some originate from Korean itself like 엉 which is the most casual way to say yes or yeah in Korean, being reduced down to just ㅇㅇ. Or ᄋᄃ from 어디 (where), ᄆᄎ from 미쳤어 (crazy), etc. By using the most prominent elements of the word to make up the shortened texting version. While others like ㅇㅋ are based on the English okay and ㅂㅂ for bye bye.
Some Korean texting slang is not based around any word though, and more what they visually look like; such as ㅠㅠ which is closed eyes crying, or ;; which looks like little sweat droplets.