Korea has moved most arriving international travelers onto a mandatory digital E-Arrival Card system, submitted online in advance of arrival rather than filled out by hand on a paper form during the flight. The system asks for the same broad categories of information the old paper card did — passport details, flight information, accommodation address in Korea — but is completed through an official government portal ahead of time.
What this means for you
Fill it out before you fly rather than assuming you'll sort it out on the plane or on arrival — build this into the same pre-departure checklist as checking your entry requirements. As with all entry procedures, confirm the current process on Korea's official immigration site close to your travel date, since implementation details can change.