How Qawwali Is played?
Typically, there are a lot of people involved in a performance, with eight or nine players making music with harmoniums, tabla, dholak and other percussive instruments. There was a time when women were banned from performing Qawwali music, but things have changed.
A lead vocalist and a small team of side singers take turns weaving in several different melodies, some of which will be improvised. After a long, improvised introduction for the main song, the singers all join in and vie with each other as the song builds towards its crescendo and sudden, abrupt end.
https://www.bbc.co.uk/programmes/articles/5Plm8bBlBd7wXjZN2zdb8Fm/a-beginners-guide-to-qawwali-music