Why have I been charged for a download when I have subscribed to Apple Music Membership

You shouldn't be unless you downloaded the song / content from the Apple Music Store.

Remember, Apple Music is a streaming service that allows you to download songs for offline consumption. But as soon as you cancel your Apple Music subscription you will lose access to all your Apple Music library and any songs you downloaded from Apple Music.

But you can still "buy" songs if you want to - and nothing stops you from doing so if you are an Apple Music subscriber.

Sounds like, perhaps, you accidentally ventured into the Apple Music Store (using the iTunes Store app) and bought a song versus downloading from Apple Music (using the Music app)

Download music to listen offline form Apple Music: https://support.apple.com/guide/iphone/add-music-and-listen-offline-iph0cff2d191/ios

Request a refund for content that you bought from Apple: https://support.apple.com/kb/HT204084

If you still need help contact Apple Support: https://getsupport.apple.com/?locale=kb&PGF=PGF63005