How does health insurance work in the US?

I am a non-US citizen and need this information to do a case. Specifically: 1) Is health insurance compulsory for everyone? 2) What happens if someone cannot afford it? 3) In the event that a medical procedure needs to be done, does health insurance cover all the bills? Does the patient need to pay anything extra? 4) Does the patient have any say over what kind of procedure he can take? Say if 2 treatments are available for his condition, can the patient choose the more expensive treatment? And if so, is it covered by the insurance? Thanks for reading this. Your help in answering any part of the questions would be greatly appreciated! Thanks to those who have responded so far. I would like to further ask: Does a health insurance contract state that it will only cover the "normal" rates for a procedure? For eg. if there are 2 possible treatments for a disease, 1 of which is more expensive but more effective than the other, will the patient only be covered by the LESS expensive one? Or is it a case in which the patient can opt for the more expensive one and "top-up" the difference? This is a crucial question to my understanding the case. Thanks!