Your primary care physician is the one who has the whole medical chart and does your yearly physicials, sees you for medical reasons such as stomach upsets, the flu, sore throats etc. Your Pdoc, at least where I live, sees you for your mental health issues only and mine won't even discuss any other health problems with me.
Many PCP will order Antidepressants unless they feel they are not qualified as in tough cases, such as mine was. If the antidepressant my PCP ordered had continued to keep working and I was pretty sx free I would not have needed a Pdoc. As it was my med pooped out and the Pdoc is your expert on mental health medications.
Hope this is clear as mud. Hugs