Air source w/ furnace running fan only

We have a standard forced-air gas furnace, maybe 15 years old. We have an air return in the living room.

Question: if we turn our thermostat to "fan only", does the system only circulate the air in the house? (i.e. only pull air through the return?) Or does it also pull in outside air?

My understanding is that furnaces use outside air for combustion, right? So is it also pulling in some of this air when the fan's running but there's no combustion?

Reason I ask is that we've got an air quality alert right now due to forest fires. So we've got the house sealed up. We'd like to run the fan for air circulation (and to run the air through the furnace filter) but want to male sure we're not pulling more outside air in.

Thanks!