Neither are you paying with USD even if your debit card directly debits your USD account. It's a proxy for paying with USD, just like a bitcoin debit account is a proxy for paying with bitcoin. That the conversion and account adjust ment happen in one currency or another does not mean that currency is directly involved, it is merely a convenient stand-in for the number reflecting your balance, an attribute of it.

People think that banks hold their money: they don't. The bank simply owes you your money but they don't hold it for you. What you get in return for your money is a statement from the bank what debt they owe you and a device to convert that debt into goods whenever you feel like it. That device is not the same as the underlying money, it is merely a proxy for your debt.

Which you'd find out about in a hurry if your bank ever went under and your card stopped to work.