It's client specific and my phone requires whatever can unlock the phone and chrome requires either windows hello or a pin if under linux.
Certain implementations do whatever, and as far as the backend is concerned, there's no way of knowing, unless you want to get into the business of locking down specific vendor keys...
But I say MFA is overrated versus just getting away from generally crappy password factors. Also passkeys are less phish-able than OTP type solutions.