Do I understand how Bitcoin Debit Card/ Direct deposit accounts work?

I've been doing a good amount of studying, maybe I just need to dive in and try it. But I'm still unsure what the general functionality of Bitcoin/crypto debit cards is. Whether the funds you have available for spending on your debit card are actually held in BTC (or another crypto) or are held in cash, and you merely have the opportunity to get BTC rewards?

What I envision is, (with round numbers for simplification). I have my paycheck of $1000 direct deposited as BTC, assuming (for simplicity) BTC is at $10,000, I now have .1 BTC, which if I spend anything from the debit card is equivalent to $1000. But anything I do spend is automatically spent from my BTC holdings rather than from some separate dollar holding.

If, continuing the previous hypothetical, without spending anything the value of BTC increases to $20,000, I now have the equivalent of $2000 that I could spend on my debit card.

Alternatively if BTC went down to $5,000, I would only have $500 available to spend on my debit card.

Obviously there are differences from card to card, different rewards, different fees etc. But is the functionality I describe how they generally work? If not, are there any apps/cards to achieve this?