Does AD B2C offer in-app authentication instead of jumping to b2clogin.com

Solution 1:

When you set up an identity provider for sign-up and sign-in in your Azure Active Directory B2C (Azure AD B2C) application, you need to specify a redirect URL, as this is the provider of trust.

You can not reference your own sites (or login.microsoftonline.com) in your applications and APIs for authenticating users with Azure AD B2C. You can use B2C in-App, but on some point in the authentication process, the microsoft authentication will take over. This is to make sure, that your app is not interfering with the users credentials. Which is the point of doing B2C.