Can someone explain how this aspect of medicine works?

This isn't about me, but I've read in a lot of articles and books and from people in real life that they want to become doctors to work with patients who can't afford life saving surgeries. They believe that health care is not an exclusive club and that everyone deserves quality, life saving health care. Everyone believes that we should all have good health care, that doesn't make us all qualified to be doctors. I've always seen this to be a very admirable approach, but how does it work? Someone still has to pay for the $500,000 surgery whether it's the insurance company or someone else. How does one go about "working with poor patients?" I mean, the doctors don't set the prices for surgeries and equipment, they're doctors. I am genuinely asking to know how this works.