How asp.net identity framework create the OTP and verified?

Question

I have a question regarding ASP.NET identity framework. Identity framework allows generating OTP code if two ways authentication is enabled! My question is, how OTP is generated and where it is saved for confirmation.

Generating Token for changing Phone Number

var code = await UserManager.GenerateChangePhoneNumberTokenAsync(User.Identity.GetUserId(), phoneNumber);

For Verifying:

UserManager.ChangePhoneNumberAsync(User.Identity.GetUserId(), model.PhoneNumber, model.Code);