I have 3 month free trial subscription of Apple Music and every time I buy a song I am been billed and my money is being deducted from my bank account at any purchase. I want to know the reason for the deduction.
Assuming you mean Apple Music, that is a streaming service. You can stream songs for free. Buying the song is completely separate and you will be billed for that. So instead of buying the song, just play it in Apple Music.