@chernobylime Aircraft are subject to defensive fire Just like ground units, so in your scenario the attacking aircraft fires, and then the defending infantry fire, regardless of there being any AAA present. The only unit that cannot fire back at aircraft is the submarine. As for Anti Aircraft Artillery, they only fire once before the conventional combat begins. AAA may fire one shot per attacking aircraft, up to a maximum of three shots.
So in the scenario you provide if the Germans attack the Russian territory with say 3 Planes and the Soviets have 3 Infanty and 1 AAA then the AAA will fire its three shots, the attacker removes any planes that are hit, and then the battle begins. AAA may be taken as casualties but do not fire again.