Do medics need flight cert to work on a helicopter or is it just required by most employers

?

I know that you have to have some special knowledge and change how you do some things on a helicopter, and some even have an expanded scope of practice, but my question is could a medevac program have its own in-house training for paramedics or is it basically a legal requirment for all helicopter paramedics to have flight paramedic certification - it looks like there are a few organizations that "certify" flight medics so which one is considered official?