I don't know what the history is, I'm sure it's complicated.

You've got hospital ambulances, private ambulances, city fire departments, county fire departments, volunteer fire departments and federal fire fighting organizations all potentially in the same place, all summonable by a 911 dispatcher. They all have EMT training and gear. Who shows up and how is it decided? Beats me.

Well, as long as whoever shows up has the required experience and equipment to solve the situation, great! I guess the fire fighting orgs have medical equipment and training too?

Sometimes human organisation is baffling if you are seeing it from a step further out but makes complete sense when viewed from the inside.