How did us troops make a difference in the final battles of the Great War?

Mostly psychologically. When the US soldiers first arrived in France, the German army (and the German war economy) was already exhausted and no longer able to adequately support, rest, or replace its soldiers. The French and British were already successfully pushing back the German lines. Moreover, the French and British had battle-hardened veterans as soldiers, the Americans raw recruits. They did not play a really major part in the final battles of WW 1.

But the arrival of the US troops convinced the Germans that the Allied forces now would have an even greater and continuing supply of replacement soldiers, guns and other material. And this for them was the final straw in a war that was already going wrong for them. At the same time, US President Woodrow Wilson was the only one of the Allied leaders who offered an in German eyes 'reasonable' peace plan.