How to have strong authentication for users using SAML in Entra External ID External Tenant?

Question

On the documentation here, there is no custom authentication or SAML based authentication for External tenants. However we have previously used B2C for many of this kind of cases. Users will have username and password (these work of course fine already). But we need every person to authenticate via strong methods (eg bank or government) to add unique PIN for each user and recognize who the user actually is.

What is the best practice to add SAML authentication and adding claims from there to the user information?