Seoul looks made for cars rather than pedestrians. Barriers often complicate crossing wide roads, but public transport is a good way to commute around.

Getting from Airport

Trains to Seoul are paid only in cash; the ride was 1200 won. There is a special card for a slightly faster express train that departs from its own platform.
They claim to refund a deposit when you return the card in some mystery shop at the airport that nobody knows where it is. Quite deceptive service design.

Getting around Seoul

Daily tickets (up to 20 rides) are only available at the airport and Seoul station. Otherwise, you need to pay by ride. Shorter rides are cheaper if you scan the card when leaving.

Mobile SIM

SIM cards at the airport are OK; we bought also a data eSim from Nomad, which worked as well if you don't need voice calls. Finally, we did not need to use the voice number at all. It might be handy to have it to book a temple-stay for example, but you'd need to buy it in advance in order to discuss the booking. We fortunately connected with the bookins by e-mail.
All operators have similar coverage, and it's OK across the country

ATMs

Most ATMs are in English, except for the distant areas where we had to use a Google translator camera.

Maps and accommodation

Naver map is must to efficiently move around. Google maps doesn't provide driving directions, only public transportation. There are also tourist trails with photos similar to street view on the main treks.
There is even more in the Naver maps if you let Google translate it for you on the web, but to book an accommodation or order a taxi, you'd need to have a local ID, so for most it's probably just to survey prices. 

Mapy.cz are best offline for trekking.

We used AirBnB and Booking.com because it was easier.

Cuisine and sights

We got used to the local cuisine quite fast, just the Korean barbeque was exceptional.

Most sights are poppy like a Disney land that tries to overwhelm your attention as fast as possible with all the bells and whistles on a weekend before you get back to work.