Use passkeys to sign in to apps and websites on iPhone - Apple Support

"Passkeys are more secure than passwords, because they're uniquely generated for every account by your own device and are less vulnerable to phishing. And they work on all your devices that are signed in to the same Apple Account. 

Also, websites and apps can automatically create passkeys for your account, so you can use the passkeys the next time you sign in. 

Like passwords, passkeys are encrypted and stored in your iCloud Keychain, where they aren't visible to anyone (including Apple)…"

https://support.apple.com/guide/iphone/use-passkeys-to-sign-in-to-apps-and-websites-iphf538ea8d0/ios