The whole “USSR allied with the Nazis” thing is actually Nazi propaganda and historical evidence proves this repeatedly. Before the war started, Stalin offered to send 1 million soldiers preemptively to England and France, together with artillery and aviation, if they agreed to a mutual defense agreement against Nazis. The soviet union wasn’t prepared industrially for a war like that, again as proven by the 20+ million deaths in the war, and wanted to postpone it as much as possible, and join the allies as soon as it started, but France and England were too eager to see communism destroyed and didn’t care about mutual defense, especially England. The fact that the Soviet Union later invaded some countries to the east of Germany was in preparation for war, to prevent Nazism from rising in these places and the military there allying with Hitler, as Finland did for example (there were plenty of Finns sieging Leningrad).
Equating Nazism and the USSR is a revisionist, fascist talking point based on purposeful misinterpretation of some data like the Molotov-Ribbentrop pact, and outright omission of other data such as the attempted Collective Security policy attempted by the USSR since the early 30s to protect Europe from fascism, that England and France conveniently didn’t agree to since nazis and fascists were enemies of communism as they were.