Source: my father served in communist army.
Maybe some people are siphoned off from the labor pool to be trained in actual warfare, but most aren't.
Eating edible grasses/herbs (even ferns), very weak bone-based soup/tea, gutter fish, frogs, roadkill, rice roots/tubers, tree bark (the NKs have been known to make pine bark "cake"), wild mushrooms, oysters/clams (if near the ocean), wild berries/fruits, lichen, bugs, rats, wild hemp (extremely nutrient rich and unregulated in NK), and sparrows, all of which are plentiful in NK. Knowing Koreans, they probably also have family gardens where they grow garlic and shallots (relatively easy to do), provided that they can retain viable seeds. I'd imagine that these private gardens are frequently stolen from by other hungry people, so they may be clandestine a lot of the time. No kimchi, though-- they can't grow the cabbage.
You see this kind of food pyramid a lot during famines-- most recently, during the Cambodian genocide but also during the Great Leap Forward in China and the Holodomor in Ukraine. Scavenging low-density calorie sources from wherever, with increasingly lowered standards for what constitutes food. The problem becomes when effectively everyone is scavenging, which leads to sparse finds and increased calorie usage during harvesting. I assume they have been in this state of extremely scarce scavengeables for years now.
There's effectively no access to dairy items or fresh fruits from what I can tell. This probably results in near-endemic calcium deficiency, and is supported by their tragically reduced heights and high infant mortality.
During the last NK famine in the 90s, there were widespread rumors of abduction of children for the purpose of cannibalism, and people were told not to eat meat unless they knew the source. It's tremendously sad.
If you want related books to help think about what sorts of things happen during these times, I'd start with Maus. It's not at all related to NK, but it captures the desperation and ingenuity of long term crisis.
Also some of them might have chicken or small garden at home, but wont admit it, because it could be illegal or whatever.
I don't doubt it, but it's probably worth noting that those North Koreans inside the army still live better than those outside it. The official ideology of the modern North Korean state is songun (https://en.wikipedia.org/wiki/Songun), "military first," meaning that the military's needs are prioritized when allocating the nation's extremely scarce resources. (Which makes sense, considering how critical the army is to maintaining the regime's hold on power.)
So if soldiers are only getting two potatoes, that doesn't mean that they'll be looking enviously upon the diets of the civilian population. The civilians will be having to make do with even less.
Eritrea also seems to be solidly in the running for this questionable record:
In the United States, individuals join the military as a way to serve their country with honor and pursue new career opportunities. But that’s not the case in Eritrea, where men and women risk their lives and those of their families just to escape military service.
Both are nation-states though, and both have governments that pass laws and then punish the masses for disobeying them.
In other words, the people of both countries have rulers, and the main difference is in the rulers' degree of "benevolence" towards their subjects.
You're fine with your particular government ruling over you because you think they would never do what Eritrea and North-Korea are doing.
But that's how German people thought before Germany turned into a mass-murdering Nazi-Germany, so that kind of thinking is clearly misguided.
Any nation-state is just a police state waiting to happen. Then it does happen, and years or decades pass until tyranny subsides, and then people go back to "it could never happen here!"
How crazy is that?
Diet early in life has the determination if you are going to have a stunted growth. By the time you finish high school you are about as tall as you are going to be.
If people join after high-school, how does that work?