Ummm, Finnish forces took part at Leningrad.
https://en.wikipedia.org/wiki/Siege_of_Leningrad#Finnish_participation
Stalin did actually invade Finland. The only reason the Russians stopped was because the Finnish fought the Red Army to a stalemate, negotiated an armistice (the September 1944 Moscow Armistice) and then joined the Allies.
It should be remembered that China was absolutely communist in areas during the war - the Communists and Nationalists were fully engaged in combat in 1927, long before the Japanese invaded.
The USSR didn’t have the resources to spend on mainland Asia until it was clear Germany was prostrated; additionally, it was agreed at the various conferences that the USSR would continue to concentrate in Europe - remember, the policy was Germany first, with Japan as a side show.
You seem to be remembering a lot of things about WW2 that just weren’t so.
Did you read the article at your link?
“The Finnish forces crossed the pre-Winter War border on the Karelian Isthmus by eliminating Soviet salients at Beloostrov and Kirjasalo, thus straightening the frontline so that it ran along the old border near the shores of Gulf of Finland and Lake Ladoga, and those positions closest to Leningrad still lying on the pre-Winter War border. According to Soviet claims, the Finnish advance was stopped in September through resistance by the Karelian Fortified Region;[43] however, Finnish troops had already earlier in August 1941 received orders to halt the advance after reaching their goals, some of which lay beyond the pre-Winter War border. After reaching their respective goals, the Finns halted their advance and started moving troops to East Karelia.[44][45] For the next three years, the Finns did little to contribute to the battle for Leningrad, maintaining their lines.[46] Their headquarters rejected German pleas for aerial attacks against Leningrad[47] and did not advance farther south from the Svir River in occupied East Karelia (160 kilometres northeast of Leningrad), which they had reached on 7 September.”
The Finns did NOT fight the Soviets to a stalemate; the Soviets wanted back what they had taken in the Winter War, and they got it. There is no way Finland was going to stop the same army that was taking all of eastern Europe, all the way to Berlin.
The whole point in Asia was that Soviet weapons armed the communist movements, and when China fell, Chinese weapons armed the communists in North Korea and Vietnam.