Hmm, 500 million is a lot but it is only 500 million. Unclear how they can finance what is already there, pay for food and schools, a nuclear weapons program, a relatively giant military with ridiculously long military training, etc, and on top of that build a whole new city out of nothing. At the end of the day China would rather not have NK collapse on itself, and I think it's pretty clear they've been extending a generous helping hand over the last few decades.
Oh for sure, both Russia and China are propping up NK in a major way. I asked myself the same question you asked when I saw 4:55-5:05 of that video, I don't believe the doors and tile shown are manufactured in NK, they look identical to the stuff we have here in South Korea, and it's all manufactured in China. They're both hiring many North Koreans who I presume get trained and shipped back to NK, providing funds, and providing building materials.