Here's an interview with a German ace who evaluated several Allied planes (he preferred the P-51):
https://www.historynet.com/aviation-history-interview-with-w...
Britain mostly was on the lookout for fighters and fighter/bombers with novel antennas, since they could be used to understand new radio navigation and radar electronics. (One of the most advanced landed near shore, but the Navy guarded it in salt water instead of handing it over for timely study.)
During the Korean War after a MiG-15 was "donated", the issue was how to verify it wasn't booby-trapped before entering it, since test pilots are valuable (Chuck Yeager was one of them.)
https://en.wikipedia.org/wiki/Operation_Moolah
At the end of the Vietnam War, the north captured an entire fleet of US aircraft (87 F-5s, plus more types) that were used successfully against air battles with China.