Now I know that there are many reports of this, but my case is different. So please bear with me
When I create WebApi on the .Net Framework and select individual user accounts, it already substitutes the working code for external authentication. 
There is AccountController, Startup.auth.cs and other code files for external authentication 
Now I know that things have changed for the .net kernel. But net.core web api in individual user accounts does not resemble anything.
When I try to do the same with WebApi on .net Core, it provides me with only one option to select any Azure application. In the figure below, with the WebApi on.net database, this drop-down list, indicated in blue, provides only one option, which is the Azure AD b2c application. There is no user account controller or other configuration files for external authentication.

There are no files that I need. 
I tried these links: https://docs.microsoft.com/en-us/aspnet/web-api/overview/security/external-authentication-services
https://docs.microsoft.com/en-us/aspnet/core/security/authentication/social/
But the first link is for the .NET Framework, which does not help me. The second only works for an asp.net core web application, but NOT WebApi
I need external authentication in the web api because I want all my users from the Internet or mobile device to be authenticated by Google or facebook. Anyone please point me in the right direction. I missed something. Are there any documents that can help me?