Yes, you need to apply for a K-ETA.
However, those who have applied for K-ETA exemption from the U.S. military authorities to the Ministry of Justice before entering the country do not have to apply for K-ETA.

No. Members of the United States Armed Forces (US Forces in Korea) do not have to apply for K-ETA under the Agreement on Facilities and Zones and Status of the United States Armed Forces in Korea under Article 4 of the Mutual Defense Treaty.

When issuing a ticket, the U.S. military must present an ID card, not a passport, to the airline and proceed.

No. Diplomatic/offcial passport passport holders can enter Korea without a K-ETA.

No, if you currently have a valid ABTC card, you can issue a flight ticket without getting a K-ETA.
However, citizens of the U.S. and Canada should be issued a K-ETA.

Yes. If you have to go through immigration to pick up your luggage and then go through the ticketing process again, you need to get a K-ETA.
You can write down the address of the international airport for the address and phone number.

(ICN) - Incheon International Airport

(Address) : 272 Gonghang-ro, Jung-gu, Incheon, Republic of Korea

(Tel) : +82 1577-2600

(Postal Code) : 22382

(GMP) - Gimpo International Airport

(Address) : 38, Haneul-gil, Gangseo-gu, Seoul, Republic of Korea

(Tel) : +82 1661-2626

(Postal Code) : 07505

(PUS) Gimhae International Airport

(Address) : 108, Gonghangjinip-ro, Gangseo-gu, Busan, Republic of Korea

(Tel) : +82 1661-2626

 (Postal Code) : 46718

(CJU) - Jeju International Airport

(Address) : 2, Gonghang-ro, Jeju-si, Jeju-do, Republic of Korea

(Tel) : +82 1661-2626

(Postal Code) : 63115

(CJJ) - Cheongju International Airport

(Address) : 980, Ochang-daero, Naesu-eup, Cheongwon-gu, Cheongju-si, Chungcheongbuk-do, Republic of Korea

(Tel) : +82 1661-2626

(Postal Code) : 28142

(TAE) – Daegu International Airport

(Address) : 221, Gonghang-ro, Dong-gu, Daegu, Republic of Korea

(Tel) : +82 1661-2626

(Postal Code) : 41052

(YNY) - Yangyang International Airport

(Address) : 201, Gonghang-ro, Sonyang-myeon, Yangyang-gun, Gangwon-do, Republic of Korea

(Tel) : +82 1661-2626

(Postal Code) : 25042

(MWX) - Muan International Airport

(Address) : 970-260, Gonghang-ro, Mangun-myeon, Muan-gun, Jeollanam-do, Republic of Korea

(Tel) : +82 1661-2626

(Postal Code) : 58533

No, you don't have to apply for K-ETA if you are a transit passenger who doesn't enter Korea.
However, if you have to go through the immigration checkpoint to find your luggage and then go through the issuance procedure again, you must get a K-ETA.

Yes, you have to apply. If you want to visit Jeju Island, you must obtain a K-ETA before boarding an aircraft or ship.

No. Anyone entering the country as a sailor or flight attendant does not need to be issued a K-ETA.

No. Those with a Korean alien registration card do not need to get an additional K-ETA.

No, those who have a Korean visa do not need to get an additional K-ETA.