What is a nation-state? Why is Korea technically not a nation-state today?

first part: A nation-state is a state that has one centralized government (making it a sovereign state), the majority share the same culture, and their political boundaries and cultural boundaries line up with each other.

(It gets more in-depth political wise but that's the basic idea of it)

second part: Korea is technically not a nation-state today because it is two separate states. North Korea is actually considered more of a cult than a nation-state because of the isolation and sanctions set in place.

not sure if that is the answer you were looking for, if you want me to explain it more in-depth or differently let me know :) hope this helps