takes a pilot to teach a pilot. don't know if they have an actual aircraft, tho.
But who has taught the initial pilot then?
Instructor+flying simulator and experience on the real one.
Most mercenary pilots are former members of regular armies. E.g in Mali many of the pilots are Ukrainian mercenaries, in the EritreanEthiopian War Russian mercenaries flown the planes on both sides (!), but you can find South Africans as well all across Africa.