The passkey is stored in the secure enclave which is protected by your unlock code & biometrics (for fast unlock), or whatever the equivalent is for Google. You're not replacing your secret key / passphrase, it's just a different way to authenticate and decrypt vaults.

It's really more a convenience on a handheld device where entering a passphrase (mine is 20 characters) is a pain in the arse. Though, if it were available for macOS, I would use it there too.