Most doctors graduating from medical school can go right into a family medicine practice without any additional training but the vast majority continue their training during a residency whey they specialize in a specific branch of medicine like surgery, internal medicine, emergency medicine, etc. That takes another four to five years after med school.
A chiropractor is not a medical doctor. They have a three year training where they learn how manage the neuromusculoskeletal system. They fall under the category of Complimentary and Alternative Medicine along with acupuncturists or hypnotherapists and the such.