What makes you say they almost won? They certainly did not. They could’ve taken Moscow and it would’ve made no difference. The USSR had way too many people, people who really didn’t want to be taken over by Nazis, and way too many resources from the US/UK for Germany to overpower them. And Germany was doing extremely poorly on resources (especially oil and steel) near the start of the war – the entire reason they invaded the USSR to begin with was because they didn’t have enough oil to meet their demands, and they knew they would collapse without seizing the USSR’s oil production/reserves (unfortunately for them, that was never going to happen). The British cutting them off from African oil made the issue signicantly more urgent. Germany also had an inferior navy to the UK, not to mention the US, with their only advantage being the large amount of submarines they had. They couldn’t realistically project much power outside of where they had land control, and crucially couldn’t protect imports from Norway, Africa, and Asia enough to make a big difference.
Germany practically signed their own death warrant by the time they invaded France. They just didn’t have the resources or arguably even the manpower to sustain that kind of war, even when controlling most of Europe and a large portion of Africa.
I’ll give some numbers to help visualize: During WW2, Germany’s peak oil production was 71,000 barrels per day (1944), mostly synthetic oil from coal. For comparison, the United States’ peak oil production was 1,875,000 barrels per day (1944) and the USSR’s was 700,000 barrels per day (1941). Germany’s peak steel production was 29.3 million tons (1944); the United States’ was 89.6 million tons (1944). The USSR produced less, about 8.5 million tons at peak (1943), but they also received about 400,000 jeeps, 7,000 tanks, 5,000 other armored vehicles, 12,000 aircraft, and a bunch of other supplies totaling up to about USD$150 billion adjusted for inflation, so steel wasn’t really much of an issue. Comparing populations, Germany’s was 69 million. The US’ was 132 million and the USSR’s was 190 million.
Considering that, it may become easier to see why Germany had absolutely no chance against the USSR in the long run; taking major cities doesn’t capitulate them. They fought tooth and nail to keep Germany from obtaining Russian & Ukrainian/Belarusian resources, as is famous from using scorched Earth tactics. It was pretty much impossible to successfully invade the USSR almost like how it’s impossible to successfully invade the US.