Ukrainian forces have captured a lot of modern Russian weapons and military equipment and made these discoveries available to Western countries that are supplying Ukraine with modern weapons and economic and diplomatic pressure on Russia.
This loot includes largely intact Iskander short range ballistic missiles, new EW (Electronic Warfare) equipment that had proven effective in Syria and Ukraine and new Azart combat radios and associated equipment. Some defective Islander missiles were recovered largely intact, which allowed close inspection of the missile design and the countermeasures Russia often spoke of but never provided details of. The countermeasures were, as expected, small decoys deployed as the Iskander came within range of the targets, as well as Western ABM (anti-ballistic missile) systems like Patriot, Thaad or the naval Standard missile defense system. Now that there were undamaged examples of these decoys available, Western ABM systems can be modified to defeat them.
The situation is much the same with captured EW and communications equipment. Another valuable item captured on the battlefield was the state of Russian training and the ability to maintain modern equipment. The most modern Russian vehicles and weapons were used in Ukraine and the Ukrainians and NATO experts had been jointly studying them intently since 2014, but from a distance. Once the Russian equipment was captured you could reach some useful conclusions about their operation. This was enhanced when Russians who used this equipment were also captured. The findings so far presented painful, for the Russians, revelations about how ineffective their expensive military reform and modernization efforts were. Examining Russian vehicles, both tanks and other armored vehicles as well as military trucks, revealed design defects and the traditional Russian inability to perform regular maintenance during peacetime or even when preparing for large scale exercises or combat. Some largely intact Russian helicopters were also examined as well as the wreckage of Russian fighters and transports. Some of these were designed and built in Ukraine before the Soviet Union dissolved in 1991. The Ukrainians learned from these bad Cold War habits and offered to refurbish older Russian aircraft and other equipment to eliminate some of these design flaws. Russia thought they had done the same, but they had not.
Russia also discovered that its SEAD (suppression of enemy air defenses) tactics and equipment did not work very well against the air defense systems the Ukrainians were using, many of them older Russian models, like the updated S300. Control of the air over Ukraine is still disputed and Russia has not been able to muster sufficient SEAD efforts to change that. Ukraine is succeeding in persuading NATO to turn over the dozens of MiG-29s some of the new East European NATO members still have. The Mig-29 owners are willing to do this but the U.S. has been blocking the effort. The Americans finally seem to be convinced that this will prolong Ukrainian efforts to prevent Russian warplanes from freely operating over Ukraine. NATO, including the Americans, are receiving a steady supply of secret components taken from captured Russian ground-based electronic warfare equipment and vehicles, plus downed jet fighters, and that is a major plus for NATO air forces. The Ukrainian Air Force would like something in return, like those old MiG-29s as well as spare parts and ammunition for their aircraft.
All this is news the Russian leadership does not want made public, if only because many Russian commanders and the Russian public were unaware of how bad it is and getting worse.