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.
Just above the idle discharge hole is the idle transfer hole, or slot. As you accelerate, there is a moment when the engine needs more air fuel and the accelerator pump circuit hasn’t kicked in yet. As the throttle valve opens, this transfer hole is uncovered and the vacuum of the intake pulls extra air fuel from this port.
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. Blow through each passage on one end and feel the other end for air coming through.