What does NASA buy from SpaceX?

As a keen observer of the intersection between space exploration and private industry, I'm often left wondering: what does NASA, the forefront of space research and development, actually purchase from SpaceX, the trailblazing private spaceflight company? From the reusable Falcon rockets to the Crew Dragon spacecraft, SpaceX has revolutionized the way we think about space travel. But what specific services or technologies does NASA acquire from SpaceX to further its own mission objectives? The question begs for an in-depth exploration of the symbiotic relationship between these two pioneers in the quest for the final frontier.