Help Understanding the Process

I put in for 6 different conditions, had my C&P exam, and received my DBQ from all of them. All but one of my exams came back as service connected, had a nexus (solid according to AI), and diagnosis.

My question is with the diagnosis. Does the examiners diagnosis hold much weight during the rating? Is it the same as if I had come in with a diagnosis from my PCP?

Could a rater deny my claim based on a short history with a diagnosis, or one that was given during the exam?

Thanks!