I'm not entirely sure about that, actually! The outdoor AC unit is what COOLS the air, and vents the heat back outside, but in our house, both the AC and the heat are apparently forced into the house through the furnace. (It doesn't make much sense to me, but the furnace does come on when the AC unit kicks in, it just isn't generating heat.) Then again, our furnace also apparently incorporates a dehumidifier - we had to have it serviced for water in the motor because the drain tube was clogged - so maybe it's both in one, and I'm just confused!"
The issue you're having here is in seeing the furnace as the entire HVAC system, not just the heat generator. In a system like yours, there are two units in the system. One is the furnace, which makes heat, and the other is the A/C unit, which is a compression coil in the system and an external exchanger that sits outside. As a side note, the exchanger doesn't cool the air (that's handled by the compression unit that's built into the system itself, inside the blower box), it gets rid of the heat that the compressor pulls out of the coils. Both of those units share the blower and duct work. So, the "furnace" doesn't come on when the A/C kicks in, the blower does, which blows the cold air around the ducts. In the winter, the furnace does kick in to heat, and then the blower moves that heated air around the ducts.