Come to think of it, would Visa/MC be doing the conversion? I believe that's the issuing bank's job. But they offer the rewards, not Visa/MC, so it doesn't change the original answer much.

Visa/MC are doing the conversion, and that does makes sense to me given their central position in the transactions. There are (basically crooked) originating (I think that's the right word) banks/payment processors that convert it for you, with a 3% plus spread. Paypal is the most egregious in this category.

The rewards are funded by the banks via interest income. 3% cashback cannot be covered by interchange fees, for instance.

Visa/MC do the conversion, but it’s not unusual for banks to tack on extra fees. Additionally banks can choose to settle in multiple currencies, and handle the FX themselves.

VISA/MC do offer the spot rate. But the idea of a spot rate is a little iffy anyway, as it assumes that there’s a counterparty willing to sell/buy at that price, and at the volume you want to transact at. When you’re operating at MC/VISA scale, that isn’t a given, and neither of them are interested in taking on FX risk. So their spot prices will be close, but likely not identical to spot prices you might find at other FX providers. Over the long term though, I would expect any delta to balance out.

When I took my Schwab debit card to Europe, the exchange rates that I got were identical to the ones posted on Visa's website, suggesting that it's Visa doing the ForEx conversions.

A lot of banks do charge a percentage fee on top of Visa/MC's exchange rate, but my bank does not.

Schwab and Fidelity debit cards are the only ones that do this AFAIK. In addition, you get unlimited ATM fee rebates, even outside the US. I’ve stopped taking cash home and instead withdraw from my Schwab account :)