so planes can’t shoot subs if there is enemy Destroyer presant in same sea zone in the whole game, or is it just in this only situation with the AC.
Enemy destroyers don’t keep planes from hitting subs. Friendly destroyers enable planes to hit subs. If there is no destroyer friendly to the firing air unit in the battle, the air unit’s hits cannot be applied to subs. This is always true, whether for attacking or defending.
Okay, Reversing the question my original question, if you’re attacking AC with fighters on board, do you use your fighters on AC in battle as well…?
You move your own fighters independently of the carrier. They take off before either unit moves, so they both begin movement in the same sea zone. Both fight in any battle(s) they end up in, which could be the same one or different ones.
Fighters on your carrier that belong to another power on your side stay on the carrier when it attacks and are treated as cargo. They may not fire and may not be taken as casualties. If the carrier they’re on is destroyed, they’re destroyed along with it.