That’s how iOS Safari’s Password AutoFill works for both iCloud Keychain and third party password managers. Password managers can also supply a prebuilt list so that Safari handles everything, although custom widget handling user input is also allowed.

https://developer.apple.com/documentation/security/password_...

Edit: Of course it’s not limited to Safari, it works for in-app authentication flows too provided apps integrate it.

Or, even better, no passwords and just sensible pki