Trane fan "On" for outside air?

Concerning the electric thermostat that controls my Trane furnace and
air conditioner, if I change my fan switch from "Auto" to "On" (so
that it runs continuously), is it pulling in air from outside or just
circulating the air already in the house?  In other words, is running
the fan a good way to cool the house at night when the air temp.
outside is a good 15 degrees cooler than inside?

Thanks.