I have it on very good authority that China does not support NK. There is no open border between China and NK. The border between the two countries is a desolate place.
China has grown tired of NK asking for more and more assistance and implying that NK helped defeat Japan during WWII.
it’s not open, but stuff goes back and forth and a certain number of people go back and forth via checkpoints. some are able to cross illegally. saw a documentary devoted to the border topic.