I want to create web api for web app and android app. I need to somehow authenticate users. Unfortunately, Microsoft doesn't talk much about that in documentation and when I want to create new project, this is what I see when I want to create app with Individual User Accounts (seems like Azure-tied service).

I don't care about how it will work, credentials may be stored in my database, may be using Google Accounts. It just need it to be safe and to work. Where should I begin? How can I make that?