The manufacturing of weapons was running 24/7 and there weren't enough Germans to keep up with demand so they used around nine million prisoners. It were these slave laborers who helped Germany from surrendering for over two years.
It's absolutely amazing what Germany accomplished thus the reason when some people talk about the best products, they mentioned German engineering. The attitude they gained during WWII towards innovation and quality, made them the leader in a variety of product categories for decades after the war.