The memoir of a Japanese fighter ace (“Samurai!”) describes fighting China’s air force in the late 1930’s. The author states that these planes were flown by foreign volunteers. Is this true? If so, who were these pilots, and how were they recruited?

You might be interested in this thread on the Republic of China Air Force with answers from u/ParkSungJun and u/unclesamsbrother. A lot of the aircraft and pilots provided to the Republic of China were Soviet and American (ETA although strictly speaking the Soviet pilots flew in the 1930s and the American Flying Tigers served starting in April 1941).