contracts on contracts on contracts; where do they come from?

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Newbie question. On the podcast I hear the word contract used a lot, whether it's in reference to a wholesaler "putting a house under contract" or a flipper "honoring a contract with a general contractor." Where do the formats for these contracts come from? Is there a boiler plate setup for your state that you just reuse as needed or is it something you have to pay a lawyer to draw up each time? Or can you just write up an agreement on the back of a napkin and call it good (exaggeration, obviously)?