Authorization through VKontakte, what is the logic of work?

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Hello. A question. For example, there is a site with VKontakte authorization. The user created a site account and linked a VK account to it (i.e., the token is already saved in the database).
How to proceed with authorization?
For example, the user is logged out and wants to log in via VK authorization (without entering the site password).
What actions/requests need to be performed on the server side to check the user and authorize him on the site?

Answer 1

Read the VKontakte API documentation.
You check that the user vk/idXXXXX is visiting you, for which you make a request to the VK API, which confirms this.