How the Russian won the world war 2?

Mainly because the Russians tactically retreated back and back more into the center of Russia, until the Germans were disconnected by their supply routes and caught by the extreme cold. And then they advanced again.

Secondly, the Russians had more soldiers than the Germans. It is said that they even had more soldiers than weapons, so sometimes they sent multiple soldiers with just one gun to take over when one of them was killed (which were many).