Apple Pay (Apple Wallet): which is the default card and does that vary?

Today, I was setting up a payment method via Mobile App. Let's say for example it was McDonald's (it wasn't).

Previous comments did not adequately address this "problem". The generic response is one card is the default card.

To elaborate/clarify:

The 'McDonald's' app asked me what payment method I wanted to use: Apple Pay or another method.

I became confused at this point. I have about 7 credit cards within my Apple Wallet.

Whenever I use any of them for a payment at a POS (Point of Sale) terminal, my iPhone says Apple Pay and that credit card I used.

Depending on my needs and acceptance, I use different payment methods setup on my iPhone. I do not have Apple Pay linked to my bank account, and I do not have an Apple Credit card.

To clarify , I needed to make an electronic payment on the spot so I didn't want to risk any issues so I manually entered one of the credit cards that I have stored in my [Apple] wallet.

As such, I'm confused what would've happened when I selected Apple Pay? Would I be provided an option to choose which credit card (payment method) I can use when setting up my Apple Pay profile on the app? And how would I go about changing it?

I know my Apple Wallet has a primary Credit card and I know my iTunes account has another (Amex) for which my Apple Pay subscriptions are being charged on.

I'm thinking my Amex card is the default Apple Pay card but at a POS it's a Mastercard branded card which I selected in my Wallet when I scan but that really makes no difference as the card that shows up first is the last one I used via wireless NFC.

Please don't reference Apple links on this because I already looked at them.

Thanks