Air pollution from China along with thinking about other people besides yourself helped with that. When the pandemic broke everyone already had a box or two of N95 masks in their house. The government also took immediate control over the mask industry and started rationing them.

They had strict mask regulations, people stayed home, they closed most businesses that would help spread it, things like home delivery for groceries was already a pretty big thing in Korea at the time, and the delivery food business is massive there. Initially, outside of an outbreak caused by a church cult, numbers were very low in Korea.

They only started going up when they allowed the kids to go back to school, but were still generally very low because people were pretty careful. They also had public free government testing and home test kits were pretty easily available. If you tested positive on a home kit, you just walked over to the local outdoor testing center, stepped up and got tested, they texted you the results the next day along with instructions on how long to quarantine and then they sent you a care package of some food, masks, blood oxygen monitors, etc.