1Password works natively with iOS... In the standard iOS keyboard when you are in a password field, click on the key icon that pops up and you can select to fill the password from 1pass.
Must be brand new, since I just set up an iPhone 7+ on the latest OS and I got a password autofill suggesting from Firefox Lockwise on the system Apple ID prompt (not during the setup flow, it was an alert dialog from Settings, if I'm remembering correctly).