I went through a building design construction course. Community college level, once a week for 12 weeks if I remember.
On the furnace heating system picture it this way. Your house is an insulated box with insulation for the outside walls only and on the ceiling. The furnace fires up and the heated is recirculated within this tightly insulated box to bring the air up to your set (comfort) temperature.
Heated air will cool on its own mainly conduction loss with some loss through leakage. The (heated) leakage air loss has to be replaced by outside air. The best route for this outside air to enter the house is to duct outside air into the return air duct so that it is heated first before it joins the circulation.
It is very dangerous to over seal the house, ie make it too airtight, and not have this outside fresh air intake. The reason is that heated air normally rises up the chimney. But when the room air cools air contracts and it will suck air (in the reverse direction) down the chimney. This suction includes toxic burnt gases present in the chimney and can be blown back into the house. You breath this stuff. This is slow poisoning.
Another reason for the fresh air intake is that in the absence of one the heated air inside the house will be drawn up the chimney together with the furnace gasses. You lose expensive heated air up the chimney every time the furnace fires up. The best solution is to have a separate fresh air supply to the furnace burners. This way outside air goes direct to the burners and goes up the chimney immediately after combustion. The burners do not use up warmed room air.
Often all the contractors do is to have an open duct from the outside to an opening near the burner assembly. Of course in winter this lets in freezing cold air. Many homeowners plug this duct up with fiberglass or remove the duct altogether. It will work but you will be sending warmed room air up the chimney. I fabricated an enclosed duct to trunk the duct air into the furnace. Cold air is drawn in only when the furnace fires up, a per normal convection, and the cold air never gets into the basement. I understand newer furnaces already have this feature.
To recap there should be a fresh air duct from the outside and connected to the furnace return air trunk. There should be a separate fresh air duct to supply fresh air to the furnace burners.