Writing my own custom back-end in Django

Question

I am trying to build an application which consumes a third party API. I want to implement OAuth2 in my application using that API.

Now, I am able to receive the authorization token, followed by an access token and refresh token. But, I am lost after this! How do I use those tokens to authenticate those users in my application?

I could not find any tutorial regarding this. Please help me here.

In a way, I want to know how python-social-auth implements the authentication back-end. That would also help, I guess.

Thank you!