How to log in through social networks with a strict separation of frontend and backend?

Question

Social media

Actually there is a front and a back.
the front interacts only with the backend, the back gives only api and is responsible for all business logic.
Everything works, it was necessary to fasten authorization through social networks.
Please tell me the best way to organize this. After all, the front cannot log in through social networks (there is no backend layer between the front and the api), and if you log in through the back, then the user will return to the back and will see bare json.
Actually, what to do and how to act?