How to delete a pass key from my Google account on iPhone?

Have you tried the steps Apple provides in Use passkeys to sign in to apps and websites on iPhone - Apple Support

Delete a passkey

1. Go to the Passwords app  on your iPhone.
2. Tap Passkeys, then tap the account for the passkey you want to delete.
3. Tap Edit.
4. Tap Delete Passkey.

Ruby2sday Said:

"Deleting Pass keys: I’m trying to delete a pass key that I mistakenly put in my Google account"

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Troubleshooting Passkey Deletion:

If you can't delete it on this device, then enable Passkeys in iCloud Settings on this device and others, and then remove the passkey from the device: Make your Passkeys and Passwords Available on all your Devices with iPhone and iCloud Keychain – Apple Support