How did the Russians win WWII?

First it helps to understand the relative sizes and strengths of the great powers in 1941.  Today, we have just two, maybe three superpowers, powers so much bigger and stronger than ordinary powers that nobody dares mess with them.  Back then, there were more great powers (Germany, Russia, Great Britain, France, USA, Japan) and they were closer to each other in strength.  Germany was the strongest and scariest European power, the US lacked the regard that it earned during WWII.  Certainly Hitler didn't think much of America. 
   Hitler attacked the Soviet Union in June 1941, a bolt out of the blue attack.  Hitler poured in 3-4 million soldiers, 5000 tanks, 10,000 warplanes.  He caught the Russians by surprise, and captured most of western Russia, encircled and destroyed two huge Russian armies, taking 1.2 million prisoners of war.  The Luftwaffe wiped the floor with the Red air force.  That first year the Germans nearly captured Moscow.  Advanced German patrols got as far as the Moscow trolley lines and claimed to have seen the domes of the Kremlin gleaming in the sun.   By the end of the year, the Germans owned the Russian heartland, all Stalin had left were a bunch of remote frontier districts.
   Somehow, I've never read a good description of just how, the Russians hung in there, drafted  another 30 million men into the Red Army, picked up and moved a thousand factories from western Russia to remote Ural Mountain locations, got production of war machines going again from Siberia, and next summer at Stalingrad met the German army head on and beat it in a standup fight. 
   To do this, Stalin's regime had to have political control via the NKVD, and a lot of popular enthusiasm for the Great Patriotic War.  Otherwise, those 30 million draftees might have decided that service in the Red Army was suicide and bolted for the woods.  And just how did the Red Army turn draftees into combat soldiers so quickly?  And trying to get a tank factory which had been dumped by the side of the railroad tracks back into production must taken superhuman effort on the part of the workers.  Suppose the workers had lost heart and just leaned on their shovels? 
   In short, it took a miracle for the Russians to stay in the war, and fight back so effectively, after the tremendous damage the Germans did to them in that first summer.