I am implementing my own custom UserStore for web api. I would like to know where I can find the source code for AspNet.Identity's UserStore because I don't intend on changing it completely but I mean to introduce additional behavior that is related to my problem space.
Here is the source code for the interface and the class from the asp.net core on GitHub
I am not sure you will find a documentation for it, but it is not hard to understand. You can always ask for an explanation here on Stackoverflow.
You can also look at this question for a stackoverflow member who is doing something like what you are doing now. MVC 5 Custom UserStore