In case of idle, the engine throttle is fully closed therefore there is no air passing through the venturi. Because of very high vacuum or depression (-5psi) inside the intake manifold between the engine cylinder and the throttle the very small idle mixture opening is positioned downstream the throttle.
With the engine cranking or at idle, the fuel (red color) from the bowl gets draws upwards through the main fuel jet and an opening to the bottom of the main air jet (emulsion tube) to the cavity where the plastic plug is located. Here it mixes with the air (blue color) draws through the idle air jet located on the outer face of the carburetor. The mixture of fuel and air (orange color) travels through the horizontal passage all the way near to the front face of the carburetor where the idle opening is positioned. Here it enters the intake manifold on its way to the engine cylinder. Due to low cylinder temperature and high amount of residual exhaust gas left in the cylinder, the idle mixture is very rich.
See movie below: