What happens when you 'signup with facebook'?

On many websites nowadays you will find the buttons of signing up or logging in using FACEBOOK, or GOOGLE or TWITTER, etc displayed. But what really happens when you signup using tech giants like Google or twitter or facebook? Today I will explain to you in simple terms. If you are having an account with facebook or google or twitter, they have some basic information about you. This information is like your name, your profile picture, your email address and probably your phone number. Before I proceed, allow me to explain an important term here: API-Application Programming Interface.

API in layman's language is a way of sharing information between applications. Facebook has the information I enumerated above about you stored in their database and this website/application you're trying to signup to needs that information about you. So, an API acts as a messenger. Facebook/Google/Twitter, etc have an API that shares information about you on your consent. When you click on signup with facebook/twitter/google button, you're redirected to the facebook/google/twitter page to login. This is just to authenticate that you're the one giving that application consent/permission.  After logging into facebook or google or twitter, they will know that you have given them consent to share your information with that website you're signing up with Facebook from and so it will redirect you back to that website and their API will give that website or application the details they asked about you Like your name, email address, phone number, profile picture among other details which are automatically saved in that website's database. That's what happens behind the scene without your knowledge. So, it's always good to read the level of access, some application may need an access level of accessing your tweets or facebook posts or posting on your profile, etc. Now you know!