For conversions between crypto and fiat or one cryptocurrency and another, we're using several providers:
Where available, we use rates from Chainlink
If Chainlink is not available, we use rates from Cryptocompare
For Crypto-to-Fiat payments, we use rates provided by Kraken and Open Exchange Rate, depending on the currency
Good to know: Request and its partners protect you against depegs (devaluations of stablecoins so that the exchange rate isn't close to 1:1). Learn more here