With a passkey you would have one in keychain that unlocks 1p. So you will only be asked for your phone password/unlock code whenever iOS asks, but it will be used to unlock 1P. Never need to enter your 1P passphrase.

I still don’t follow. So instead of 1 password that you remember, there would now be a passkey?

I feel that is too risky. Where would you store that passkey? And what if you lose it?

I suppose the 1p emergency kit could print out a QR code or something.

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.