Ambulances almost never have a doctor onboard in the US
There are 2 main levels of EMS, Basic Life Support (BLS) and Advanced Life Support (ALS) There's some additional sub-categories, and some things vary according to local regulations and such, bit I'm just giving a very general overview.
As a general rule BLS will be staffed by EMTs and ALS will have a paramedic.
Paramedics can start an IV, administer some more medications than an EMT can, and provide some other more advanced treatments, but they stop well short of what a doctor can do.
Not all ambulances are staffed to the ALS level, a lot of places have had a hard time keeping their EMS agencies fully staffed, I know in my area, in theory, all of our agencies can provide ALS service, but frequently some of them have to go BLS-only due to staffing issues. Because of this, some areas have started expanding the scope of what EMTs are able to do to make up for a lack of paramedics (though they also often face a lack of EMTs as well)