[deleted] so the yubikey has different features.
There is an option for "short press" and "long press" of the button.
You can type in your own diceware password in the yubi key "long press" option. (That way if you lose it, you can manually type it in yourself)
Then when you plug it into the phone, or computer, you press the button and hold for a second, and it then rapidly types what ever you have set.
So what you do is...
When you set your phone password....you decide to manually type a short easy one....followed by pressing the yubikey. Because the yubikey acts like an external keyboard this means you can have a much longer password without the inconvenience of having to type it in manually
This means, with the yubikey on its own only provides part of the password so you can't gain access to your phone with JUST the yubikey as you would need to type in your manual password beforehand.
Does this make sense?