I would first check to see if you are on a "Transit Exclusive Domestic Flights" TEDF. Last I checked it was only KE1401 and KE1407.

If it is TEDF, then the flight is treated as an International flight. So you just do a standard international transfer in ICN, and then clear customs in Busan.

If it is not one of these flights then you have to do the usual song and dance of clearing customs in ICN then going to the domestic flight terminal.

Edit to add: You can probably just ask at check-in if your Busan is a TEDF or a domestic flight. Not a big deal either way, ICN prides themselves on some of the fastest immigration and customs process. You can usually get through in under a minute or two, other than waiting for bags.