So googling says this is available to apply for after 5 years for Spain. Which suggests that the health care benefits may not be substantially better for quite some time after moving. Especially if moving from the US and giving up several tens of thousands of dollars per year in salary.
The last part of this sentence makes thing increasingly unclear-sounding:
> You have to prove that you have adequate financial resources to provide for you and your family (if applicable) – such as pension, scholarship or salary – and proof of public or private health insurance with a company authorised to operate in Spain.
Why would you need health insurance of your own if you'd end up on a state plan?
You can't just take the first plane to Spain and say, "please treat my cancer for free because my country refuses to spend any money on me". This do not work like this.
Spain has health agreements with many countries concerning its citizens. Sometimes they are symmetric, other not so. If Spain will be billed if a Spaniard needs to use the public health care system in the country X, then the country X will be billed in return also if one of their citizens needs to receive public health care in Spain. As foreigner turist either your country or your insurance will pay for you, but you are guaranteed to be treated by fine physicians if you really need it.
You could need a health insurance if you are a retired citizen of other country living as expat in Spain but that never worked there, (therefore never contributed to support the public health-care system).
Because you can apply for permanent residency while being retired and living your elder days under the spanish sun, in which case you don't get health coverage because you are not (and never were) a spanish worker.
If you have been working in spain, you are covered by the spanish public health system which is your proof of public health insurance right there. Also, so long as you keep being an "active worker" (that means either actually working or inscribed in the public job-seeking people registry INEM) you will maintain your health coverage whether you are a permanent resident or not.
If you work legally (EU Shengen area, or having a work visa) with salary, for a company, you have full public health coverage for you and your family, just like every Spanish citizen (Spanish citizens have public universal health-care coverage even being unemployed).
The last part of this sentence makes thing increasingly unclear-sounding:
> You have to prove that you have adequate financial resources to provide for you and your family (if applicable) – such as pension, scholarship or salary – and proof of public or private health insurance with a company authorised to operate in Spain.
Why would you need health insurance of your own if you'd end up on a state plan?