Integrating Google + Facebook Authentication on ASP.NET Core WebAPI backend along React/Android app frontend

I've a

1. Webapp in React.js
2. An Android mobile app
3. An ASP.NET Core WebAPI

I want to integrate Google and Facebook authentication to my WebApp and Android app. User can choose either Google or FB to authenticate.

I was able to get access token from Google & Facebook with React app and Android app. Now my question is

1. How can the WebAPI know where to validate the token? From Google or from Facebook?
2. I'm confused about the way this authentication federation works

What can be the step by step process on ASP.NET server to achieve the same? I was able to do react & android part but confused with C# part.

Or should I go for Identity Server 4 federation?