how does Germany occupy European Russia (its huge) or Japan occupy China?

Japan already had experience with occupying Korea, so they would apply their same administration to China. In fact, they actually set up their own puppet state called "Manchukuo" to assist with this.

As for Germany & Russia, they would have gone the way of the "Native Americans" if the anti-slavic sentiments were to be believed. But once again, history is written by the winners. Israel did the exact same thing with Palestine, and today they're given billions of dollars just for existing. At least the Slavs would have been liberated from Communism, and the most German looking ones would have been able to assimilate very quickly.