adding UserName and FirstName in ASP.NET Core Identity

Question

I created a new ASP.NET Core Web Application with Core 3.1 and I change the Authentication to Individual User Account.

 

In Register, I need use UserName for Login. I also need to add FirstName and Last as additional fields. 

 

My problem is, I don't know how to modify Identity because I can't see the models, pages to do it.