Mozart: String Quartet # 17 in B flat major (The Hunt) - HAGEN QUARTET

Mozart's String Quartet No. 17 in B-flat major, K. 458, nicknamed "The Hunt", is the fourth of the Quartets dedicated to Haydn. It was completed in 1784.[1] It is in four movements:

Allegro vivace assai
Menuetto and Trio. Moderato
Adagio, in E-flat major
Allegro assai
The incomparable Hagen Quartet performs live from the late 1990's.
How does a mere mortal compose such music as this ?