Shared Family Payments & Apple Pay

I have had iCloud Family sharing set up with my Partner for quite some time, with my main debit card set up as the shared payment. I have since changed this to my debit card for our join account.

I have Apple Pay set up on my iPhone, and only have it linked to my main debit card - it is the only available card for Apple Pay.

If I make purchases in the App Store / iTunes - which card would be charged?