Was it bottom up, with factories competing to make products (trucks, ammo) for lend lease? Basically government used tax payer money to become a big buyer of things, incentivizing companies to reorient towards making those things? And then when the US entered the war, the types of contracts offered changed.
Was it top down, with the government operating more like a command economy telling businesses what to make, and when? It wasn't a choice, it was a demand because the country is at war!
These are two guesses I have. My mental model is based mostly on the movie "War Dogs." I'm sure it's a mix of both bottom up and top down. Maybe with a mix of cocktail parties with important industry business people and important government people chatting about would is feasible sprinkled in.
I do not understand how the war time mobilization of the US economy actually happened. If anyone knows, I would appreciate a summary or a book recommendation. I think I have a topic for my next Wikipedia deep dive.