If there is a NATO-Russia war I fully expect China to immediately act to ‘protect’ Siberia by annexing it. That’s the extent of what I expect from China.
I think the best they could hope for would be for China to attack some US-Nato allies in Asia, like Japan or South Korea. China doesn’t really have the capacity to do much more than that. They couldn’t expect China to help them in Europe any more than Hitler expected help from the Japanese in that sphere.