In Germany, every adult is required to have health insurance. If they don't have a job, the government will pay the premiums. Has been working like that for decades.
The entire health care system is paid for almost exclusively through those premiums.

It's really not that hard.

Per capita, Germans spend about $6k/annually on health care, per person. Why is it more than twice that in the US?

Germans also have literally dozens of health insurance providers to choose from, no matter where they live. Some are tied to employers, but you don't lose coverage if you change jobs.

https://www.krankenkassen.de/gesetzliche-krankenkassen/krankenkassen-liste/

Health Insurance providers whose services are part of the public mandate are Non-Profit Organizations, by law.