It doesn't, sorta? Jong-un has an older brother who is not in politics. Jong-un was selected because he formulated a state-building ideology around how his grandfather and father operated the state and their wins and misses, anyone could have done that but he was the one who did the work and became leader of the party. As I understand Jong-un was also the leader for the nuclear program as an officer in the military and this has basically earned him immense respect in the DPRK.

Jong-un is the leader of the worker's party and the leader of the military, he's actually serving two functions that could be occupied by different people, both are appointed positions though iirc leader of the military (president) has to also be elected on in a confidence vote with the masses.

Jong-un is not the leader of the legislative body, that role is selected by the legislative body (which is elected locally like a congress). The current VP of state (2nd in line) is also the appointed chair of the legislature. The legislature is the highest organ of state, though the president is the highest ranking individual.

This is not dissimilar to how other ML states have been run. It's often that the next leader is the most advanced Communist (whether that's theoretically or organizationally though you need both). Deng was educated and shaped into the successor of Mao by Mao himself. Lenin had also trained Stalin among others. Xi similarly was such an advanced theoretician and organizer that he also earned his way to the top. It's not surprising that Il-sung and Jong-il had both trained their children to be advanced communists, though it's clear that not all of their kids wanted and worked to become leaders. So I'd say it's mostly coincidence, the "fixed" aspects are being the child of the leader you are being raised by the most advanced Communist in the nation, that's certainly an educational advantage, still Jong-un's older brother is apparently just a random musician (according to an exiled statesman).