An airside economizer is an attribute of an air handler that allows the unit to utilize "free cooling" when outside air temperature (or enthalpy) is lower than return air temperature (or enthalpy).
When the unit activates its economizer mode, the outside air damper modulates inversely with the return air damper to provide a mixed air condition as close to the discharge air temperature setpoint as possible. The unit is said to be in full economizer when the outside air damper is fully open, the return air damper is fully closed, the exhaust air damper fully open, the difference in supply airflow and exhaust airflow is just the air needed for pressurizing the building, and the supply air temperature setpoint is met. This means that there is no need for the cooling coil to provide cooling - either by d/x or chilled water.
Introducing this extra outside air brings up the need for more return air in order for the building pressure to stay at appropriate levels. In order to bring back this increased airflow there arises the need for an exhaust or return fan, depending on the static pressure loss of the return ductwork (if more than say 2 inches, the fan in the return position is probably the way to go.)
I would say there are many factors in designing these systems that cannot be learned from a tutorial - it usually takes an experienced collegue or mentor to get you to the point where you understand air handler design.