As most people know, the vast majority of the fighting in Nazi Germany happened on the Eastern Front, and the Soviet/Nazi war was, if taken outside of the context of the rest of WWII, the largest war in human history. However, looking at the size of the Nazi Army, even if they are the best land army in the world, how could they last as long as they did against the Soviets? Well the thing is, there were a lot of different people running around fighting in the Eastern Front
On the Nazi side, you of course had the Germans themselves, the best trained, equipped, and experience land army in the world (at least at that time, they will lose that distinction in 43). In 41, 3,767,000 forces were fighting in the east. In addition to the Nazis, you had their allied states who fought along side them. Bulgaria, Italy, Hungary, Romania, and Finland all provided their own forces to fight in the east, and since they were extremely anti communist, they provided a great deal of troops. You also had the Axis puppet states, Slovakia and Croatia, which while not providing as many troops as the allies, provided more than the occupied territories. The Occupied territories mostly just provided resources, though they did all send their own volunteer SS units to fight, I mean of course the French, Western Poland, Norway, Denmark, Serbia, occupied Czech Republic (mostly german Czech volunteers), Belgium, Luxenburg, Holland, Greece, Albania, Bosnia, Montenegro. Then you had the Soviet occupied lands that willingly worked with the Nazis, at least until the Nazis revealed how much worse they were. THe Estonias, Latvians, Lithuanians, Moldavians, Ukrainians, Belorussians, Cossacks, and even half a million Russians who were sick of Stalinist oppression. Finally on the Axis side, you had anti communist volunteers who wanted to fight the Soviets, mostly from Spain but also from Turkey, Free France, Ireland, India, Pakistan, Syria, Palestine, Iran, Burma, Armenia, Azerbaijani, Georgians, Turkestan, Volga-Tartars, and even Britain (though only 59 British signed up to fight the Soviets). Even 18 Americans signed up to fight for the Nazis, what good it did them.
However to complicate things, there were many independent forces fighting in the Eastern Front who hated the Nazis and the Soviets. These included the Russian Free Army, about 150,000 Russians mostly former White Army troops, Nationalist forces from the Ukraine, Poland, and the Caucuses, Mongolian rebels, and several central Asian rebel groups. While they were too disorganized and disliked to really make a difference, they did fight against both the Soviets and the Nazis, mostly as partisans. In addition, the remnants of the forces that fought in the Russian Civil War, specifically the Blacks (anarchists) who had been hiding in the Urals for about a decade as mountain rebels/bandits, attempted to use the war to over throw the communists. However, cut off from any allies and the woefully undersupplied, they did some damage but were totally crushed by the Soviet Union.
In short, there were a lot of factions running around in the East, because most people hated both the Soviets and the Nazis, as they were both mass murdering dictatorships that treated the people like shit. However, the Nazis were so much more cruel, that most who didn’t like the Soviets either went independent (and were killed) or fought for Stalin anyways.