The North Koreans are not that advanced to make technology like this, I will be surprised if most of their weapons are freely given to them by the Russians.
You still doubt the North Korean technology? Let me give an example. After all these years, countries such as Brazil and Canada have failed to develop nuclear weapons. And despite having a defense budget of almost 1% of that of these nations, the North Koreans have succeeded in creating nuclear warheads. And moreover, they developed their own technology unlike the Chinese who plagiarized the Russian technology.