In my work we have the need to create an OpenId Connect Provider to integrate to another Azure AD B2C Instance (oidc identity provider).
The challenge is the oidcp must integrate with the onpremise system via API to get or validate the user's credentials. (The company doesn't want to migrate users to B2C).
It is possible with Azure AD B2C Custom Policies?
Another solution can be replicate users to Azure AD B2C using Microsoft Graph and synchronise passwords, but it's more bureaucratic in this company.