When I was learning to play the saxophone my instructor had an old copy of the Real Book. The transcriptions of songs by some of the jazz legends would blow my mind. I thought the people being able to transcribe those were geniuses (still do). They had no sophisticated software to help them. It was all done manually by listening to countless hours of recordings over and over again. All those chord changes, harmonies,…
I've spoken to a few professional arrangers, and I always ask "How many times did you have to listen to that to write it down?" "Just once." Genius indeed. Or 10,00 hours. Or both.
Mozart became famous at the age of 14 by transcribing Allegri's Miserere from memory after hearing it performed in the Sistine Chapel. It was a piece of music that had been composed about 150 years earlier, and which was not permitted to be written down or distributed, only to be performed once a year during Holy Week services in the Chapel.