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You've certainly used (or know of) a product or service where there was a "Buy" button that was actually Wyre's whitelabel "fiat onramp" service -- essentially a widget they add to their website, or a couple API calls from an app.
Mostly used for ACH (US) and SEPA (EU).
The wallets add this and earn a commission on each sale that occurs.
Others in the "fiat onramp" API space are:: MoonPay, Ramp Network, SimplexCC, Transak, Mercuryo, OnRamper, Banxa, and a handful of other smaller ones.