This is always a favorite historical ‘what if’, so here’s my two cents worth.
What would have happened if the US had stayed a non-combatant in WW2?
My guess is that Russia would have survived. Barbarossa had stopped for the winter before the Pearl Harbor attack. So long as the Japanese played their hand with Russia the same way the 1942 campaign season in Europe would not be decisively different. Past that point events for Germany would improve compared to reality.
Russia without American trucks and Germany without a second front might lead to an eastern stalemate.
Without https://ru.usembassy.gov/world-war-ii-allies-u-s-lend-lease-to-the-soviet-union-1941-1945/, the Soviet Union would probably have collapsed against Germany.