Why Korean calligraphy is our traditional Chinese character?

I thought South Korea and North Korea were collectively called Koryo in the past, and Koryo was a subordinate country of China. At first, there were no characters in Korea, and Chinese characters were brought back to Korea by Korean envoys. It was not until King Sejong was in a hurry that scholars from all over the country invented the correct pronunciation that Korea really had its own characters, and Korean cuisine began to widely use Korean, but Chinese characters have been used to this day. Up to now, Chinese characters are still used as the official language in Korea. Students in schools generally have to learn Chinese characters, and each Korean word is fixed.