One interesting point here is sanctions mainly being imposed against nations which geographically border china. Sanctions against both north korea and russia made both countries more dependent upon china. This in turn gave china leverage in trade & negotiations with both countries fueling china's economic growth.
Sounds like a conspiracy theory. There already is a simple explanation - North Korea is playing with nukes and was sanctioned for decades for all the atrocities that they commit domestically, Russia is being sanctioned over its war against Ukraine. It's not like the west has conspired with China to sanction some innocent countries to throw them into the hands of China.
Also, North Korea lives off China's money, so there's no economic benefit here for the Chinese, they are using NK as a political and military tool, plus they don't want it to collaps because it would mean millions of refugees.