Why did millions of civilians in Leningrad and the subcontinent die? Was it because the supplies were cut or were the axis putting them in gas chambers?

It was because there supplies were cut

Why did people in camps die?

Because their supplies were cut and they were starving. Starving people to death is the most easiest way to kill a large group of people en masse.

Of course you can’t understand this because you’re brainwashed with cognitive dissonance. You just think the nazis wasted resources and a large amount of time gassing them all.