We had originally planned to go all-in on passkeys for ONCE/Campfire, and we built the early authentication system entirely around that. It was not a simple setup! Handling passkeys properly is surprisingly complicated on the backend, but we got it done. Unfortunately, the user experience kinda sucked, so we ended up ripping it all out...
You do realize that your biometric authentication techniques don’t actually send your biometrics (e.g. fingerprint/face) to the website you’re using and that you are actually just registering your device and storing a private key? Your biometrics are used to authenticate with your local device and unlock a locally-stored private key.
That private key is essentially what passkeys are doing, storing a private key either in a password manager or locally on device backed by some security hardware (e.g. TPM, secure enclave, hardware-backed keystore).