It is simple to explain.

Think of on going Geo-political events like a 3 course Meal.

The Chinese know Russian Federation is only the starter course.

The main course is actually China.

After China the Pudding will be Iran/North Korea

If the Russian federation is comprehensively defeated in this proxy war by US/NATO.

And Russia subsequently regime changed with new Western sponsored Oligarchs placed in key positions in government/Energy etc.

China knows the Pacific/South China Sea will be filled by US/NATO whose attention will be fully focused on militarily/economically subjugating China influence.

China can not afford to let Russia fail and become a US/NATO vassal State.

China future is inextricably linked to Russia future.

If Russia falls to US/NATO war machine.China knows it will also be doomed to fall under the Wheels of the US/NATO war machine.