The aircraft.cfg and .dp files can be opened with Notepad. Until you know your way around more, you may only want to explore copies of these files and not the originals, in case you change something you don't want to do.
In the aircraft's folder there have to be at least these 4 folders and 3 flight files. There can also be some other files like Notes but these aren't necessary
Model: contains the 3d models
Panel: contains the 2d panel (if there is no 2d panel files then the folder and a dummy still need to be in there)
Sound: contains the sound files or at least a sound.cfg file that references a set of sounds such as the A6m2_Zero/sound.
Texture: contains all of the paint files-
any repaints go here replacing the originals.
The aircraft.cfg file: contains aircraft physical attributes and some flight performance - mainly affects the player's aircraft
The aircraft.air file: contains aircraft performance -
The aircraft.dp file: contains the damage profile and weapons performance
BTW - I would recommend unzipping the add on aircraft into a desktop folder before putting it into the CFS2/AIRCRAFT so you can make sure you have the above in the airplane's folder. Sometimes a contributor will leave out something, will reference a sound file that you don't have or even have the plane folder inside another folder that would need to be removed from before putting it into the CFS2/AIRCRAFT folder. We're all amateurs after all so sometimes things aren't quite as organized as needed.
There are 2 whole libraries of WWI aircraft here for you choose from
https://www.sim-outhouse.com/sohforums/local_links.php?catid=124
https://www.sim-outhouse.com/freeflight/WW1_CFS2.html
Lots to explore and add on