How Did North Korea Build Their ICBM?

They didn't!

“It’s likely that these engines came from Ukraine — probably illicitly,” Mr. Elleman said in an interview. “The big question is how many they have and whether the Ukrainians are helping them now. I’m very worried.”

Bolstering his conclusion, he added, was a finding by United Nations investigators that North Korea tried six years ago to steal missile secrets from the Ukrainian complex. Two North Koreans were caught, and a U.N. report said the information they tried to steal was focused on advanced “missile systems, liquid-propellant engines, spacecraft and missile fuel supply systems.”

Investigators now believe that, amid the chaos of post-revolutionary Ukraine, Pyongyang tried again.