Welcome, Bapinurnose!
Yes, they can.
I recently got A&AR, and I’ve enjoyed it so far. I know that anti-air guns and other planes can defend against air attacks, but is that it? Can any naval or land units attack fighters/bombers?
Welcome!
Yes. Unless the rules specifically state otherwise, any unit type can hit any other unit type.
Thank you, and so submarines can’t attack planes right?
Correct, submarines can not fire back at planes but can be hit by them.
Also, I had a question about submarines. When your submarines fire in the opening fire step, can they fire in the regular battle if they weren’t destroyed?
No. Subs only fire in the Opening Fire step, which happens in each combat round.