> How does one do this? Where do you learn it? Is there a book you suggest? The fact that I'm even asking this feel weird (I'm insured).
You can't do this if you're insured, because then the terms of your insurance apply. You can still get bills reduced, but the reasons are different and it's a bit more involved.
If you're uninsured, it's simple. You literally just tell them, "I have no insurance. I can't afford $45,000. It would take me years to pay that, and I'd probably default on the bill beforehand. However, if you reduce the billed amount to $500, I can pay that in full today, in cash.".
Substitute appropriate values in as needed, but that's the general idea. It's actually not all that different from what private insurers do, except they do all this up-front in bulk, so their rates are set in long-term contracts.