Russia is interested in expanding territory, access to warm water ports, and satellite buffer states.
They are civil with Red China because they border each other, and both have nukes and large armies. The USSR intended for Red China to be a Jr. puppet after the Chicom revolution, but Mao figured he didn’t have to take orders from Moscow (and even Ho Chi Minh!) and established the Communist “heresy” of “Socialism in One Country”, which is contrary to Marxism.
Allies are not the same as buddies. Common interests does not mean friendship.
Yet generation, after generation, there they are, buddies, working against their common enemy and even fighting shoulder to shoulder against it in wars.