According to their site:

https://www.koreanair.com/mobile/glo...-check-in.html

Baggage "usually transfers" regardless of airline. You'll have to check first by phone to be safe and sure. If this is the case, which is good.....then you'll just follow signs at T2 for transfer and you'll go to the desk there to get your Xiamen BP. Presumably, as the case with most Chinese airlines, expect mobile boarding pass to be nonexistent/not worth any thing haha. Xiamen departs from the Concourse since it's closer to T2. Just take the free underground shuttle train between.

If you do have to collect your bags - you'll do so at T2 and then have to go to T1 to check in - which takes 20 minutes by bus. Bus is free.

AREX train is 6 minutes BUT just as long by the time you walk down to the train station and then back out into Terminal 1...so I'd just take the bus. AND you have to pay for this.