You use two different cards, and each has its own job.
Your Monefit Card is what you pay with. It's linked to your credit limit, and every purchase you make with it uses that limit. You get a virtual card in the app, plus a physical card, and you can add it to Google Pay.
The card you add as a payment method is your own debit card from your own bank. It's the source of the money when you repay what you've spent. You can't spend your credit limit from this card, and adding it doesn't merge the two cards or give us access to your bank account or balance.
💳 So the flow is simple: spend with your Monefit Card, repay from your own debit card.
Both cards can sit in Google Pay at the same time without affecting each other — you choose which one to pay with, the same as with any two cards in your wallet.