Presumably delivered to the factory, loaded by the crew, start up and off they go - certainly as far as Stalingrad was concerned, given the proximity of the front in the later stages of the German advance.I'd want my tank bombed and fuelled up before going to battle, so where did they keep those items in the factory. But I digress.
Then again, the Soviets had some desperate ideas at the time and were not averse to throwing men away so it wouldn't surprise me if they didn't send them out as little more than an APC with a big gun; big armoured box with the crew firing their personal weapons from it whilst giving mobile cover to the infantry and relying on psychological effect and flattening anyone/collapsing foxholes that got in the way.
Who knows, really...? You hear of all sorts of weird stuff going on under the "fog of war," some of which turns out to be absolutely gospel, some of which has been "sexed up" and some of which is complete mash. However, one would hope that any decent historian would not single-source any of their research material.