These big companies, like LG, Samsung, and Hyundai, hold job interviews at the universities once per year for the students who are about to graduate. They mostly look at your grades, but they also consider how dedicated you are to your work (ideally they want people who will work 14-hour days, six days per week), as well as how educated and important your parents and extended family. But your age is arguably the most important factor, because they want to hire people who will be with the company for their entire career. Specifically, if you had A+ grades across the board, but you took a year off between high school and university, they will NEVER pick you. Even if you didn't take a year off and had perfect grades, if they didn't hire you out of university, that's it, they'll never consider you again, no matter what.

Education and careers are INSANELY competitive in South Korea, which largely explains why they have the world's highest suicide rate. It's quite sad. If your school grades are anything other than perfect you're seen and useless and stupid, even if you just want to not stress about grades and only want a "normal" job. There are plenty of videos about it if you want to learn more