The recirculate vent takes in part of the air from inside and part comes in from the outside. What fraction depends on the car and the speed.
One test is to have the car operating on high blower in dash vent air conditioning mode and then push or select recirculate. After a few seconds you should hear more noise as air is taken from inside. The return opening is above the driver'* foot.
Then you should hear less noise when recirculate is turned off.
Don't try this when first turning the car on air conditioning when the car is already hot inside if you have an automatic temperature control system; it will automatically pick recirulate for a few minutes to cool the car more quickly.
Dual auto air on both