Churchill saw that the defeat of Britain was inevitable without allied help so through the Tizard Mission they transferred their military technology knowledge to the US in exchange for help. That included the cavity magnetron radar technology, one of the first mobile radars as well as other important innovations.
The US engaged the Germans on land but that went very poorly, so they kept doing strategic airstrikes, notably taking down the German synthetic fuel production capabilities. This forced Germans to advance east. Meanwhile America supplied the Soviets with trucks and boots, helping the Soviets fight the Germans, a successful plan that resulted in heavy Soviet casualties.
I'll just address the inaccuracies in your second paragraph.
By the time the US was engaging the Germans on land (Operation Torch in Nov. 1942), the Germans were already stuck in Stalingrad and had been defeated in their attempt to capture Moscow. They would never pose a serious threat in the East again. By spring they would be retreating as the Soviets unleashed their rebuilt armies on the weary Wehrmacht.
Operation Torch didn't go "poorly," though the green US troops didn't perform as well as seasoned British or German troops would have. Nor did US troops perform poorly during Operation Husky. By the time the Allies landed in Normandy, the US was performing extremely well.
Strategic airstrikes by the US (and British) never had a significant impact on the German industrial output. Nor did they have a damaging effect on German morale.
The US engaged the Germans on land but that went very poorly, so they kept doing strategic airstrikes, notably taking down the German synthetic fuel production capabilities. This forced Germans to advance east. Meanwhile America supplied the Soviets with trucks and boots, helping the Soviets fight the Germans, a successful plan that resulted in heavy Soviet casualties.
The Pacific front was a different story.