There is a truly excellent series of Podcast from the BBC called "The Lazarus Heist", explaining in detail how the cyberwarfare capacity of North Korea is constitued, and what kind of attack they have done; I cannot recommend it enough.

The short version: they take their best math-inclined students, and send groups of them in China, to learn about computer hacking (and the world at large, to know what or who to hack). The group aspect serves as a self-surveillance check, to minimize risks of defections.