The Chinese Air Force has established a training unit to accurately (as possible) portray enemy (especially American and Indian) aircraft and combat tactics. Thus there are three Blue-Army Aggressor Squadrons (Blue is the bad guys in Chinese training, Red is the good guys) for this. One is equipped with Su-30s, to represent American F-15s or Indian Su-30s. Another has the J-10A, which is similar to the F-16. The third squadron has J-7s (Chinese copies of the MiG-21), which represent low end threats, like the many MiG-21s India still uses.
Using your own aircraft for "aggressor (or dissimilar) training" began in the 1969, when the U.S. Navy established the original "Top Gun" fighter pilot school. This was done in response to the poor performance of its pilots against North Vietnamese pilots flying Russian fighters. What made the Top Gun operation different was that the training emphasized how the enemy aircraft and pilots operated. This was called "dissimilar training". In the past, American pilots practiced against American pilots, with everyone flying American aircraft and using American tactics. It worked in World War II, because the enemy pilots were not getting a lot of practice and were using similar aircraft and tactics anyway. Most importantly, there was a lot of aerial combat going on, providing ample opportunity for on- the- job training. Not so in Vietnam, where the quite different Russian trained North Vietnamese were giving U.S. aviators an awful time. The four week Top Gun program solved the problem. The air force followed shortly with its Red Flag school. In the early 1980s, the Russians established a dissimilar air combat school, and the Chinese followed in 1987.
Over the last four decades, the two American training programs have developed differently, and the entire concept of "dissimilar training" has changed. The navy kept Top Gun as a program to hone fighter pilot's combat skills. The air force made their Red Flag program more elaborate, bringing in the many different types of aircraft involved in combat missions (especially electronic warfare.) But after the Cold War ended, it became increasingly obvious that none of our potential enemies was providing their fighter pilots with much training at all. In other words, the dissimilar training for U.S. fighter pilots was not a crucial as it had been during the Cold War. Actually, it had been noted that flying skills of Soviet pilots was declining in the 1980s, as economic problems in the USSR caused cuts in flying time. During that period, American pilots were actually increasing their flying time. Moreover, U.S. flight simulators were getting better. American pilots were finding that even the game oriented combat flight simulators had some training value. So in the late 1990s, Top Gun and Red Flag found their budgets cut. But the programs remain, as does the memory of why they were set up in the first place. If we find that, say, China is continuing to improve its combat aviation, gives its fighter pilots more flying time and their politicians maintain a bellicose attitude towards the U.S., there will be a need to increase American Top Gun training. Because of the new Chinese "dissimilar training" effort, the U.S. Top Gun and Red Flag schools are being restored to their former prominence, sort of. The Chinese move is certainly a very meaningful one, as it shows that they are serious about preparing their pilots to fight, and defeat Taiwanese and American pilots. Dissimilar training is how that is done.
Recently, the U.S. Navy refurbished a surplus U.S. Air Force National Guard F-16 flight simulator to help keep its F-16 pilots in shape for using F-16s to train navy pilots (in F-18s) how to best deal with Chinese, and other potential enemy pilots. The navy uses F-16s because these aircraft are best able to replicate the performance of likely high end enemy fighters. That's because Russia and China have used the F-16 as the model for most of their latest fighters (the Russian MiG-29 and Chinese J-10). The navy bought 26 of a special model (F-16N) of the aircraft in the late 1980s. But in the 1990s, the navy retired its F-16Ns, because of metal fatigue, and had to wait nearly a decade before it got sixteen more. The refurbished simulator had its cockpit modified to reflect the one the navy F-16s use.
The navy also uses F-5s to simulate lower performance enemy fighters. The navy has also bought and modified 44 F-5E fighters from Switzerland. The U.S. uses F-5s, a 12 ton fighter roughly similar to the MiG-21. The F-5 is normally armed with two 20mm cannon, and three tons of missiles and bombs. The U.S. Navy modified and refurbished the Swiss F-5s so their performance better matched that of Russian or Chinese aircraft.
The MiG-21 is still widely used, although it is rapidly disappearing. It is a 9.5 ton, 1950s design, the most widely produced post World War II fighter (over 10,000 built). It is cheap, and easy to maintain, but not so effective in the air. Many nations keep them in service because of the low cost, and because a wide range of avionics and weapons upgrades are available. Not really designed for ground attack, but it can carry 1.5 tons of bombs. U.S. pilots are much better at killing MiG-21s once they have trained against an F-5 being flown like a MiG-21.