It’s UN Security Council sanctions. They are not lifting existing sanctions, just not implementing new sanctions on the DPRK.
In Russia, there are two versions about why Russia participated in the sanctioning of DPRK.
First, is that Russia wanted to appease the West so they went along with the sanctions.
Second, is that China wanted the DPRK to stop developing its nuclear weapons (remember the Chinese leadership has a lot of libs who love America at one point, especially before Trump), but the DPRK didn’t listen (good decision, considering what happened to Libya) so China decided to punish DPRK from the UNSC as a warning, and dragging Russia to go along with it.
Both versions are not mutually exclusive.
They were not exactly friendly until the last few years. China was looking to deepen its economic ties with the US and EU, and saw North Korea’s nuclear development as a nuisance to their path towards prosperity. You have to understand that, at least from 2005-2015, China has a lot of pro-Western libs who love America. I cannot emphasize enough how many of my friends saw capitalism as the way forward even as late as 5 years ago.
Of course, Trump gave China the biggest wake up call they could ever hope for, and things have never been the same since.