the DPRK is a democratic country that uses democratic centralism, just like any other country.
Kim Jong Un gets elected to his roles in the state, the highest of them being the Chairman of the State Affairs Commission, by the Supreme People's Assembly, which is the highest organ in the DPRK. The Supreme People's Assembly is made up of representatives that are democratically elected in regional elections. Party affiliation doesn't matter and there are representatives from all of the three parties in the DPRK. The three parties form an alliance and rule the country together.
As others have pointed out the DPRK also uses workplace democracy through a system called the Taean Work System or Socialist Management System. The workers actively choose the leadership of their companies, by electing a management board.