Q: how does the separation of powers work in North Korea?
There aren't any.
On paper the North Korean Constitution provides things like freedom of the press and of speech but this is all meaningless, and provisions for the judiciary, but these are all meaningless in practice. Completely meaningless.
Basically, the way it works is that the military regime headed by the fat, little bald dude controls everything, for everyone, all the time. All the other provisions on paper are just there for show.
We're talking about a country whose President has been dead for a long time, and he will remain president into the foreseeable future.