How to find out at least something about a user who is logged in via OAuth 2.0 and MS Graph API?

Question

Microsoft

There is a big problem! That big problem is Microsoft. At work, you need to implement an entrance through their service / site.
It is necessary to implement SSO through MS. I dug for a long time and in many directions, from Sharepoint to O365 and Graph API.
In general, I don't even know if I'm going in the right direction. So please point me in the right direction.
Task: to implement an entrance on the site through something small-scale (Sharepoint, O365, everything that is directly related to MS). Ideal - I would like to get the request address and its parameters (required headers and arguments).
Their documentation is disgusting, machine translation. If you look at the documentation without translation, then there are a lot of words that are not relevant, everything is tied to using their products only through their products (C#, etc.)