Like is this some massive, deep plot between Russia and China? Russia starts a war with Ukraine, gets Europe and US involved, then China and North Korea throw their weight into it, Russia comes out on top, but China betrays them and take over the world.
More likely the reverse. China "follows up" in response to Russian successes in Ukraine by going after Taiwan. US intervenes to protect US (great circle) Trade Routes between the US and Singapore(Strait of Malacca) and trade routes with other nations bordering the SCS. As well as for the purpose of bolstering relations with Allies in the East Asia region (again, gotta protect those trade routes) so they don't switch sides and align with China.
US in an all-out war with China, one where Japan has also outright said they would help defend Taiwan, even if the Americans don't... And Russia gets a unique opportunity there.
Guess who has S500 batteries stationed along the potential flight path of many ballistic missiles launched from China and heading for Japan? Russia does.
Russia simply declaring itself to be "neutral" in that conflict, but closing its airspace for all involved sides then give Putin cover for using the S500 system to shoot down Chinese Missiles headed for Japan that overfly Russia.
China would obviously be pissed at Putin doing that, but declaring war on Russia over it would be very stupid on China's part all the same.
But in the meantime, the people of Japan would be left in a very interesting position where it concerns Russia at that point.
If China did declare war on Russia over it, well. Some of the history from WW2 comes to mind. Putin loves his soviet history. People need to remember what was going on with Poland at the start of the war. Russia and Germany both invaded Poland. Then the Germans declared war on Russia. Russia joined the Allies(most of whom had defense treaties with Poland prior to 1939), and at the end of the war, who had control of Poland? The Soviet Union.
If China goes after Taiwan, Putin is betting that the world(Well, the US, UK, and Japan at a minimum) will let him keep Ukraine.