Alien craft contain three key UFO Components - a power source, a navigation system and a hull built from alien alloys. The hull is specially designed to allow gravity waves to be directed and controlled. In addition a small quantity of Elerium is required to fuel the power source. Once the principles of construction are understood, and the functions of each component are known, then it is possible to build some equivalent craft.
There are three primary UFO Components and one secondary. You require varying amounts of these in order to research and build the Firestorm, the Lightning, and the Avenger. They can all be obtained after Alien Base and UFO Ground Assault missions, and where they are not destroyed from either craft weapons or the crash, from a UFO Crash Recovery Mission.
These components can be found on larger UFOs and Alien Bases in various quantities, they can be researched but provide no major strategic benefit to X-COM.