Well, once the war was over big US companies just switched their production capacity to civilian markets
It's not quite as simple as that; the US was the only major industrial power that didn't need to rebuild its infrastructure from rubble. That gave the US a huge leg up. Granted it was subsidising reconstruction in the rest of the world, but that also served to create markets for exporting to. If it hadn't been for WW2, the British Empire would still be a - if not the - superpower today.
It's not quite as simple as that; the US was the only major industrial power that didn't need to rebuild its infrastructure from rubble. That gave the US a huge leg up. Granted it was subsidising reconstruction in the rest of the world, but that also served to create markets for exporting to. If it hadn't been for WW2, the British Empire would still be a - if not the - superpower today.