Source and nature of NameIdentifier claim value

[xperiandri]

When I log in using MS Account then ASP.NET core reads http://schemas.xmlsoap.org/ws/2005/05/identity/claims/nameidentifier
https://github.com/dotnet/aspnetcore/blob/44b3ddd19216a31c9ace4b6f8b8fb74f0643be44/src/Identity/Core/src/SignInManager.cs#L673
I expected that it should be oid claim. But it is not, and it has strange format (not GUID) and value.
What is that?
And how is it calculated?
@jmprieur

I need that because I implemented exchanging a Teams token for my token. So that I want MS Account login and Teams token exchange refer to the same AspNetUserLogin record. But I can't figure out how ProviderKey is determined