Cryptocurrency debit cards aren’t really related to cryptos per se. They’re not decentralized and they are mostly based on legacy technology. The only difference being that they’re funded in some cryptocurrency, which is then converted into USD or other circulating fiat money in real time. Here’s a short overview of how these debit cards work.
A centralized entity, usually a financial institution, will hold reserves in cryptocurrencies and US Dollars (or some other circulating currency). When the client heads to a shop and requests to pay using a cryptocurrency debit card, the amount charged will be converted to crypto in real time and will be debited by the financial institution. It’s a kind of real time exchange.
As with everything in the financial world, you pay a high premium for such convenience. Your cryptocurrency balance will be debited at a rate that is always favorable to the card issuer, which is how they make money. This is extremely convenient and secure, as long as the financial backers have enough liquidity, there shouldn’t be a problem with such systems.
Remember, the current generation of cryptocurrency debit cards aren’t at all related to cryptocurrencies except for the fact they can be funded using cryptos. The entire process is a legacy financial transaction that is usually intermediated by financial networks like VISA or MasterCard.