Fuel travels from the fuel bowl through the main jet and up through the idle tube. Fuel is pulled by the maximum vacuum at idle. Air flows in through the idle air bleed. The air bleed also acts as a relief when the engine rpm is above idle. The air is mixed with the fuel and flows down to the idle discharge. The air fuel mixture is calibrated here by the idle mixture screw. The 2 idle transfer tubes are additional air bleeds at idle rpm.
The air bleed and idle tubes are small passages and obviously need to be clean & clear, or you will get a rough idle. Any vacuum leak will also cause a rough idle. Any corrosion in these tubes will be very difficult to get out and keep in mind that the current carburetor cleaners on the market may not remove the corrosion.