+1
Recirculated closes the damper in the air box and uses cabin air only.
Rear heater fan under the from passenger seat is always recirculated only.
Fresh air is indeed from under the plenum at the base of the windshield.
In the past, Ive sprinkled a whole box of baking soda all over DRY carpet as it absorbs odors. Leave for a few minutes and vacuum.
You can spray Fabreeze or something like that as well.
Also, an interior steam clean is never a bad thing.