Sometimes here on Academia I read messages referring to American National Science Foundation (NSF), "the Usa agency that supports fundamental research and education in all the non-medical fields of science and engineering" (page on Wikipedia).

In this question and this question we discussed about NSF PostDoctoral funding possibilities. Since I am in the European Union, I don't know very much about NSF.

How does NSF funding system work?

How does NSF PostDoctoral funding program work?

Does it fund only American citizens or anyone that wants to work as a researcher in the Usa?

Many thanks!