Just sharing with you my experience.
When arriving at ICN airport, the first checkpoint is where the Yellow Health Declaration Form is submitted. The person there asked if I wanted to take the PCR test at the airport or near my place in Seoul. The difference is that if you choose to take the test at the airport, you will have to stay there until the results come back (could be several hours). However if you choose to test near home, you can leave the airport and then get tested within 24 hrs or so. This was my experience as of August 2021.
Note: This is only for foreigners who have a long term visa for Korea and have been vaccinated abroad (outside of Korea). If your situation is not the same, then your requirements may be different.