I have the need to incorporate Azure AD B2C into an web app that is already working with Azure AD. I found the poor Microsoft documentation an says that i have to federate Azure Ad B2C to Azure AD (https://learn.microsoft.com/en-us/azure/active-directory-b2c/faq?tabs=app-reg-ga). next I found this article that tries to explain the process https://medium.com/the-new-control-plane/connecting-azure-ad-b2c-to-azure-ad-via-the-b2c-custom-identity-provider-42fbc2832e32 but sadly it´s not clear enough.
If I create 2 different tenants which one should I use to protect my .Net 5 API and which one should I use to authenticate my web Angular 11 App?
You have two options
I would recommend option 1. https://learn.microsoft.com/en-us/azure/active-directory-b2c/code-samples#web-apps-and-apis