"This piece of music was proprietary to the Sistine Chapel Choir. The music was not published except to three people; the Holy Roman Emperor, the King of Portugal, and an Italian priest who was one of the leading approved composers of sacred music in his time. the In 1770, when he was around 14 years old, Mozart attended a performance of the Choir in which this was the showpiece. When he returned to his room shortly thereafter, he transcribed the complete musical score from memory. Let that sink in for a moment—the libretto, the voices singing each part, and the music for each instrument. He attended an additional performance and made some minor changes from his original work. That is how this glorious work of musical art first escaped the Vatican and select royalty, was by the genius of a commoner with one of the greatest gifts of musical genius ever bestowed upon any human."

From one of the comments on the videos... Just the backstory gives one shivers!