When you first arrive at an airport, you are directed to a CBSA Officer who checks your Customs Declaration card (you will be provided with this card on your flight). When they ask "purpose of travel" and you answer that you are arriving to begin PR, they will make markings on your card and direct you into a line where the card is checked again and you are then directed to Immigration (as I recall a huge "I" was marked and circled on my Declaration card).
At Immigration, you are then processed by another CBSA Officer, who will ask you to confirm the details on your COPR, such as name, intended address in Canada, presence of any dependents not listed. You will be asked to sign next to all your answers and sign the bottom of the COPR (both copies). Your visa will be cancelled, and your passport stamped with your arrival date. He/she may also explain your residency obligation (mine did). You will then be directed to the baggage area to claim your bags.
Once you have your bags, you then go to a third CBSA officer, who examines your luggage and your B4 and B4A forms (if you have goods to follow). You will be given a receipt from the Customs Officer, showing the total value in Canadian dollars of your imported items. After that, it's "Welcome to Canada!" and off you go on your way.