There's only one real K-ETA website, from the government: https://www.k-eta.go.kr/portal/apply/index.do
All others may or may not be legit, and even if they actually provide you with a K-ETA, they'll charge you for the privilege, on top of the government's fee.
Citizens of 22 countries and regions are exempted until end of 2024. Worth checking out to see if your country is in the list.
Can you explain how you found the real from the government page and how to do this in general?
In the specific case of the K-ETA, it was easy: when I first learned about this, some airlines, including KAL, posted an announcement, with a link to the official website.
But in general, look for a
.go[v].TLD website (here, .go.kr). Another common case is Vietnam: googling for
vietnam evisa will give you a bunch of commercial websites, and one government website, with a very unfortunate domain name (because it's unmemorizable for most non-Vietnamese speakers),
evisa.xuatnhapcanh.gov.vn (Vietnamese: xuất nhập cảnh 出入境 Immigration).