Determination, and defending a lot of space for time. USSR had a lot of space to trade. Holding on until Winter helped. The Germans had some significant issues with both the space and the weather.

The Soviet military learned fast. It is often seen as some monolithic glacial structure, but organizational/political issues aside, it was as capable of learning on its feet and adapting as any other. Removing the senior personnel removed a lot of experience, but it didn't remove intelligence from the younger folk rising to fill the ranks.

The disaster of the soviet Winter War with Finland emboldened Hitler but also stimulated reform in the Red army which allowed some recovery from the earlier Great Purge. The opening of Operation Barbarossa was a disaster for the Soviets but it was largely a failure of Stalin who forbade the Red Army from responding to the German preparations. The fact that so many Red Army officers did recognise the coming attack shows that it was not a dysfunctional organisation. Once the supreme leader recovered from his nervous breakdown and began deferring to Generals like Zhukov the Red Army could bring its enormous advantage in numbers and materials to bear. In spite of colossal losses Soviet production had been so great and manpower reserves were so great that, with supplies from the UK and later the USA plugging any critical gaps, the Wehrmacht didn't stand much of a chance in war of attrition. The German strategy was to break the Red Army quickly and while some would argue they came close in 1941 it was clear after the failure to capture Moscow that the Eastern Front would become exactly the kind of conflict that favoured the Red Army.