It's going to be difficult to cover everything here. HVAC systems vary as much as makes and models do. That said, here are some of the basics when it comes to the movement of air in the HVAC system.
The HVAC assembly is located inside your dash. As we've established, it contains the AC evaporator and heater core. One makes cold air, the other warm air. The next thing the HVAC needs to do is distribute that air into the passenger compartment. It does this with a series of vents, doors, and seals. As you make different selections on your HVAC controls, you might hear different things moving around inside the dash. These are the doors I spoke of. They help direct the air to the desired location in the vehicle as commanded by the driver. These doors are sometimes controlled by cables, vacuum motors, or electric motors. If there is a problem with one of these motors or the doors they control, you won't have airflow where you want it.
There are so many different configurations of this that there is no way I can cover it all here. I can offer a couple of tips, however. The first is observation. If you believe you have a problem with the HVAC duct system, put your head under the dash and work the controls. You can sometimes see the different components move around as you do this. If you're looking for a problem, work the controls that
pertain to that problem and see if you can put your eyes on the cause. It can sometimes be as simple as a cable that slipped off.
Another thing to watch out for is obstructions. I have kids. They are probably the number-one cause of this. They think it might be a good idea to see if a pen fits in the vent, or they might have a favorite toy they put on the dash and it's small enough to fall down the defrost vent. These items can sometimes get wedged inside the HVAC system and cause problems with the movement of the doors in the system. If this is the case, you might have to remove the HVAC assembly to remove the obstruction. Sometimes you can sneak in there with just the right tool, so be sure to try that first.