Ok, here we go... ;)
As you know, textures are basically the paint job for the aircraft. You can have textures three different ways.
1. They come with the aircraft
2. You create them (see the re-painting forum)
3. You download and install them.
Inside the FS Directory is a folder called Simobjects that contains all of the vehicles in the sim. Inside of this you'll find a folder called 'Airplanes'. Each plane has an individual aircraft folder that contains the 3d model, panel, sounds, etc for that plane. You will also find a file called the Aircraft.cfg. This basically tells the sim where to find the texture folders, gauges, what to call it, and anything else it needs to interact with the sim.
Inside the Aircraft.cfg, each variation of the plane has it's own section, labeled Flightsim.1, Flightsim.2, etc...
That help? ;)