MFA - Duo
Duo is a multi-faceted provider and can only be used on your Auth0 tenant if all other factors are disabled.
Your Duo account can be configured to support push notifications, SMS, OTP, phone callback, and more. See the Duo documentation for more details on Duo setup.
When enabling Duo in the Dashboard, you will need to click on the Duo factor and fill in a few settings fields in order to link your Duo account to Auth0.
End user experience
The user will see a prompt for the second factor with Duo, listing the options you have enabled in your Duo account.