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.
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.