Here's how it works for a car with A/C.
The Non A/C car would not have the evaporator on the outer firewall behind the engine and would not have the flap or "valve" in the picture that is controlled by the left knob. The air from the fan would go directly to the heater core.
Here's the heater plenum that is under the dash and contains the heater core. It has small openings molded along the seam to direct air to the passenger and driver area. The two round holes in the bottom portion of the picture is where the defrost ducts connect (the plenum is separated into two parts in this pic)
And here is the core from the engine compartment with the fan box removed. Air is drawn from the circular hole via the squirrel cage fan, directed across the heater core in the square hole and on into the heater plenum under the dash.You can also see where the water connections are located.
And a car with the A/C plenum installed on the firewall next to the fan box.