These might help:
To include the files/ module your application need to customize, Right click your project in the solution explorer and choose Add > New Scaffolded Item, Select Identity, Add, check the files you want to customize and pick the DBContext.
Alternatively, If you want to see how the code work, for instance, Login, open the login.cshtml.cs under Areas, Identity, Pages, Account, Expand Login.cshtml. Then place a break point on
asp.net identity - Where are the Login and Register pages in an AspNet Core scaffolded app? - Stack Overflow
ASP.NET Blog | ASP.NET Core 2.1.0-preview1: Introducing Identity UI as a library