With the engine running, air is drawn in by the downward stroke of the pistons through the carburetor venturi. This creates a vacuum in the venturi pulling fuel from the bowl.
Once past idle speed, fuel flows from the float bowl through the main jet and into the bottom of the main well.
Fuel moves up the main well and past the air bleed holes in the side of the well tube. Air is added from the main well bleed.
More air is introduced from the aspirating nozzle and into fuel as it passes from the top of the main well to the vertical passage leading to the nozzle tube extension.
The throttle plates controls the amount of fuel that will enter the intake manifold, thus regulating the engine power.
Both barrels have identical passages and function simultaneously.
Obviously these small passages are critical to how the carburetor performs. When cleaning spray carb cleaner through each passage to make sure they are clean and clear.