The book does not specifically say anything.
Yes, in both cases.
If aircraft are supporting a ground attack, they can be hit by anti-aircraft fire, but the player may choose to take infantry or tank losses before sacrificing aircraft.
If the aircraft are defending a territory, it can be hit by infantry or tanks, but this will typically happen after the defender has lost all his less valuable infantry and tanks.
As a practical matter, aircraft are seldom hit. That's because the player can choose which units to take as casualties, and will usually choose infantry or tanks. Aircraft become casualties only when the player has "run out" of other units.
Yes, aircraft units can be hit by both infantry units and tank units.
Remember that a unit is not a single individual; this is not a single infantryman or a single tank. Think of a unit as a division (or corp) with all the attached artillery, half-tracks, jeeps, headquarters tents, mechanics, engineers, and light anti-aircraft weaponry required to support ~10,000 men.