7400.2P - https://www.faa.gov/air_traffic/publications/atpubs/pham_htm...
They list the air space requirements, the MSL's, the procedures, etc. 500ft AGL isn't set in stone but it's the moment when you would enter an airspace should you be in one. This is why people go out to the deserts where there's no airspace and the minimums are FL10.