After the pandemic, unfortunately the flight connection from Incheon airport to Busan ‘Gimhae’ arrport is not available. So, there are two alternatives which you can consider. You can find the information at GA homepage https://www.iauga2022.org/general/general_03.asp?sMenu=gen3
Here is an English instruction on YouTube how to take ARX to move from Incheon airport to Gimpo airport or to Seoul Train station. https://youtu.be/iYeR1bAasuc
Tip. I would take the ARX, move to Seoul Train Station and take the high speed train (KTX). It will take less time and is less stressful.