This is probably incredibly stupid, but I was wondering: how do Monzo transfer money from our accounts to foreign accounts when we buy stuff abroad, or in another currency? How do they convert the GBP into USD, or less traded currencies such as Zimbabwean Dollars?
Others will know more, but, to kick things off, this is something Monzo have entrusted mainly to MasterCard. They accept the transactions in the foreign currency and then perform the conversion at the daily MasterCard exchange rate
To follow on from what @MichaelMayes said, I believe Monzo transfer money to Mastercard, then Mastercard UK distribute funds to its relevant international versions (such as Mastercard Zimbabwe) who then in turn pay the relevant merchants. Since Mastercard are handling the transfer from Monzo, that’s how they have the Mastercard exchange rate.
That being said, since your initial question was “how do Monzo pay for foreign transactions”, they will also be paying Mastercard a small amount per transaction, which is likely to be higher for currency conversion (see: other banks’ charging fees for this).