Since World War II, most military aircraft ended up being scrapped, not shot down. Some nations, particularly the United States, have an intermediate status; storage. One such site in the United States is AMARC (Aerospace Maintenance and Recovery Center). This facility, out in the Arizona desert, stores nearly 5,000 military aircraft no longer needed for active service. Every year, some are recalled, refurbished and sent back to work. But most get "harvested" for spare parts, until what's left is sold for scrap.
AMARC isn't the only storage site, just the largest (in the world). Many other air bases in dry climates have room for some aircraft that might be needed again. The U.S. Marine Corps recently took an old AV-8 Harrier vertical takeoff fighter, that had been in storage for sixteen years at one of its air bases, and restored it to duty as a two seat trainer. The marines didn't think they would need that old AV-8. But the new F-35B, which is to replace the AV-8, is late in arriving, and operations in Afghanistan have worn down the existing AV-8s. So reinforcements have been called up from storage sites.
This points out one of the major problems with modern warplanes; that some models have remained in service far longer than anyone expected. This happened partly because modern aircraft are built to last, and used engineering advances that worked out better than expected (engineers tend to overbuild when they can).
Commercial transports are very sturdy beasts, as they have to fly up to 12 hours a day for weeks at a time. Military aircraft fly less often, although their sturdiness is also meant to deal with the violent maneuvers of combat. But heavy bombers and transports can go on and on, as they don't fly as much as the civilian transports and the basic technology they are based on hasn't changed much. The best example is the B-52 bomber, which entered service half a century ago and the ones still flying were built forty years ago. There are also over a hundred, 70 year old, DC-3 civilian transports still in the air as well.
Most warplanes are in production for a decade or less. Once the manufacturing stops, it starts to become difficult to get spare parts. The tools and equipment used to make the aircraft components are usually scrapped. Making the parts from scratch is so expensive that it is often cheaper to scrap aircraft and buy a new design. But a new aircraft is often more than the budget can bear as well. The solution to this problem is cannibalization. That is, using some aircraft, either those wrecked in accidents or older models retired to the "bone yard", just for spare parts. This has been a practice in combat from the very beginning of military aviation. Especially during World War I, when more aircraft were lost to bad landings and takeoffs than to enemy action, the wrecks became a source of replacement parts for airframes and engines of aircraft still in service. Thus the most efficient bone yard in the world is Americas AMARC. While some of the aircraft stored there are recalled to active service every year, all are liable for disassembly to provide parts for aircraft that are still flying.
Over 4,500 aircraft are stored at AMARC, and 500-2,000 spare parts orders are filled each month. Not just for American military aircraft, but for those of allies as well. Australia keeps its 1960s era F-111's flying with spare parts from old U.S. and British F-111s stored at AMARC. The U.S. Air Force A-10, built in the 1970s, and not a popular air force candidate for a new model, is kept flying (because it's so damn useful) with parts from AMARC. Even when parts are still in production, a wartime surge, as was experienced during the Afghanistan campaign, will outstrip the manufacturers ability to produce them. In this case, AMARC delivered parts for the F-18.
AMARC was set up in 1985, consolidating bone yard operations there and from other locations in the United States. In that first year, it delivered spare parts worth half a billion dollars. While the airframes, stripped of all their more valuable parts, are worth only about 25 cents a pound as scrap, some of the parts are worth their weight in gold. Engines, which often comprise a third (or more) of an aircraft's value, are the most valuable single items. And each engine consists of thousands parts, some of which are worth quite a bit, even if the engine is no longer in use by any aircraft. Other nations cannibalize their retired or obsolete warplanes, but few have organized the operation as efficiently as the United States.