I can't really figure it out. Someone told me once, but basically it is different in all US states. Some states have a coroner system, others do not. Our state doesn't have one as far as I know.
In states with coroners, the coroner is responsible for determining cause of death, whether an autopsy is needed, and contacting a pathologist to perform such a task. A coroner is not necessarily a pathologist (or even a doctor). It is often an ELECTED position requiring nothing more than a high school education and the ability to convince people to vote for you. Thus, these people are often former cops, judges, probably some doctors, but they could be a gas station attendant who talks a good game. They are responsible for documenting cause of death, etc, trends in the county they work in. Possibly more of a legal thing than a medical thing. But clearly some knowledge of medicine would help.
From what I can gather, there are some turf wars, because coroners don't have to listen much to the pathologist doing the autopsy. Many coroners, who have been at the job for many years, are obviously experienced and know what they are doing. But many probably have not as much of a clue.
Perhaps someone who has worked within a coroner system can help.
In states without coroners, medical examiners do everything, and are not elected.