I think the two most important differences between US war preparations and German war preparations in WW2 were (1) The US prepared for war fairly openly, while Germany conducted most of the preparation in secret, and (2) The US was prepared to exert economic superiority over a prolonged war of...
I think the two most important differences between US war preparations and German war preparations in WW2 were (1) The US prepared for war fairly openly, while Germany conducted most of the preparation in secret, and (2) The US was prepared to exert economic superiority over a prolonged war of attrition, while Germany was unprepared for a long conflict and hoping for a swift victory.
Germany was ostensibly not allowed to militarize under the Treaty of Versailles (signed at the end of WW1), so they had to conduct most of their military preparations in secret, disguising them as much as possible. It wasn't until the late 1930s that they really openly began to militarize, and within a few years they had already started the war by invading Poland. Hitler had a plan to win the war in four years, and only established sufficient preparation to support Germany's military for that length of time. Their plans didn't even make it four years, and Germany was already strained in supplies and struggling to maintain its advance against the USSR by 1942.
The US, on the other hand, stayed out of the war until the end of 1941, when Japan launched a surprise attack on Pearl Harbor. The US declared war on Japan in response, and Germany declared war on them in response to that. Before that, the US had been covertly supporting the UK (and to a lesser extent, the USSR) in the war effort, as well as quite overtly expanding their own military capability, particular in terms of naval forces. The US also instituted a draft even before officially entering the war.
Though formally neutral, US leaders recognized that they'd be brought into the war sooner or later, and wanted to make sure they were ready. And ready they were: By the time the US actually entered the war, they were already militarily and economically superior to every other country in the world; victory was largely a matter of time as the overwhelming US might was brought to bear against dwindling German and Japanese forces.
Still, in part because the US waited so long to enter the war, losses were heavy among other Allied nations, especially the USSR. The USSR lost over 20 times as many people as the US, and twice as many as Germany. The UK lost almost as many as the US, despite having a much smaller population to begin with.