How to create an authenticator like Google or VK?

Question

OAuth

Hello. I have 2 web services. How to make sure that in one of them, users have the opportunity to log in through an existing account in the second? The same functionality is implemented by various buttons in authorization forms, for example, "Sign in with Google", etc., a window pops up where they ask for permissions and enter. Naturally, I don’t ask for a specific answer, I ask you to suggest what technology needs to be studied and how to move forward?

Answer 1

You either need to have a common database of users ...
Or so that by clicking the button - enter through - a check for a user is sent to the database where this user is

Answer 2

specific answer https://identityserver4.readthedocs.io/en/latest/