It completely depends on the depth of the model and how much goes into rendering and handling events on the model. Most times to save CPU cycles the models are made with as few polygons as possible, with textures for details. There's a fine balance between the number of polygons and size/resolution of textures.
Let's look first at a perfect world situation: inifinite computing power. In this case the models could be made as detailed as possible, with every single piece and part placed into the final object. For example, you could render hydraulic lines, control cables and fuel tanks, then place those inside the modelled aircraft frame (which itself is individually modelled) and so on. Each of these pieces would be interactive and have effects on other pieces of the plane. Therefore, when something like a 20mm cannon round hit a wing the exact damage and systems failures would occur in the sim as they would in the real world.
Unfortunately, since we don't have infinite computing power, compromises must be made. These include limited polygons, limited interactivity between components, and limited ability to alter flight characteristics based on damage. We can see this in the current damage model of AH. In simplified terms, an aircraft is broken down to certain components such as wings, fuselage, tail, etc. Each of these may contain other parts (ailerons, flaps, etc). Each of the base components has a given amount of damage it can sustain before it fails. Let's just call this "hit points" for the sake of discussion.
Each of the weapons does a certain amount of damage (ie, takes away a given number of hit points) per strike. Once a part's hit points are drained to 0, that part fails. In the case of AH, when a part fails, it is simply removed from the model of the plane, and flight characteristics are adjusted accordingly.
Enough of that and back to your question about primitives and wire models. Usually these are used in tandem with each other (again because of limited computing power). Wire models are easy to build and manipulate, and don't cost that much computing-wise. The primitives are completed objects (such as spheres, cones, etc) that already have given characteristics associated with them. So, in a flight sim, you could model your plane with a wire frame and use that for control points (such as how to change the model when flaps are raised and lowered). You could then use the primitives to represent the objects (like the flaps).
