What is not covered and why?
This book doesn't cover Razor Pages, SignalR, gRPC, and Blazor.
The reason is that gRPC and SignalR are already very specialized, and don't really need to be customized. Blazor is a new member of the ASP.NET Core family and is not widely used yet. Also, the author isn't familiar enough with Blazor to know all the screws to customize it. Razor Pages is on top of the MVC framework and customizations for MVC are also valid for Razor Pages.