Grandson purchased app via Purchase Sharing

This morning an invoice popped in detailing my grandson's purchase of a subscription to YouTube Premium!

I like to think of myself as an experienced Apple customer but I had missed this purchasing element when setting up family members with Apple Music.

I have now asked for a refund and deleted my grandson from the family group.

There doesn't seem to be a way to offer individual members of the group a purchasing option like my wife and grandmother.

Happily they can all still access Apple Music.

Can an experienced user clarify the position?

Is Purchase Sharing 'On' by default when signing up family members to apps like Apple Music?

Can individual family members be permitted to purchase?

In Purchase Sharing it says that any apps or perhaps music/films previously bought using my card will no longer be accessible to the original user. This is why I want to allow my wife to purchase - she doesn't want to lose any items in her music or video libraries.

This must catch out a lot of people!