The participants in operation warp speed get paid whether or not their vaccine makes it through trials.
To use your analogy, farmers can sign contracts for "all the apples that's produced this year" if they want to offload the risk of crop failure rather than a contract for a specific number of apples.
A `firm order` is a business term to mean an order which is non-cancelable. It would be correct to say Pfizer was only tangentially connected to Warp Speed as Warp Speed money was used to place the firm order of $2 billion doses with Pfizer.
Pfizer knew for a fact, even if COVID was miraculously cured, or it turned out there was a super cheap treatment, or anything that made their vaccine worthless, they could still sell $2 billion units to the US.
To use your analogy, farmers can sign contracts for "all the apples that's produced this year" if they want to offload the risk of crop failure rather than a contract for a specific number of apples.