The ac isn't taking air in from outside, it's two different (and isolated) air routes. It takes in air from inside the house, cools it, and puts it back into the house. The outside air loop takes in outside air, heats it, and dumps it back outside.

These two loops never mix, at least on a residential system. Commercial is a different beast that does use external intakes.

Not unless your house is like a space habitat. Homes have tons of gaps that air can seep through.