Question about unit structure and a couple of roles.

Hi all - I'm a writer and looking for some information to try and be as accurate as possible.

Are door gunners a thing? If so is it RAF, AAC or FAA - I'm guessing it's not an entry level role as can't find it on the website for recruitment. Does anyone know length of service/rank required.

On a basic level - what is the command structure of an Army battalion (C/O, 2ic, intelligence, etc what are their ranks how does it break down to company and platoon level) and also the same but for the Marines, I know language is slightly different.

With the Navy - what would the crew make up of a Destroyer look like?

Thanks in advance.