In ASP.NET Core, the recommended way to store configuration settings for different environments is by using appsettings.json files. These files can contain environment-specific configuration sections, allowing you to maintain settings separately for Development, Staging, and Production environments without modifying the code.

Attribute routing allows you to specify route templates for actions directly within your controller using attributes like [HttpGet] and [Route]. To distinguish the two methods, you can use attribute routing like [HttpGet("{controller=Products}/create")] for product creation and [HttpGet("{controller=Products}/details/{id}")] for displaying product details, providing clear and distinct routes for each action.

In the context of user registration in ASP.NET Core, validation primarily ensures that users provide a valid email address. This is important for sending account confirmation emails and maintaining accurate user information in the system. It's a critical step in verifying the authenticity of user accounts.

Test-Driven Development (TDD) is a software development methodology where tests are written before the actual code. Developers write small, focused tests that guide the development process, helping ensure that the code meets the requirements and is thoroughly tested.

The "Startup.cs" file in an ASP.NET Core project is a crucial part of the application's configuration and initialization. It defines routes, configures middleware, sets up services, and performs other app initializations. It is the entry point for configuring the ASP.NET Core application pipeline.

The project.json file was used in ASP.NET Core 1.0, but it was replaced with a different project file format starting from ASP.NET Core 2.0.

To enable MVC in ASP.NET Core, you should invoke the services.AddMvc() method inside the ConfigureServices method of the Startup.cs class. This method configures MVC services, such as routing, controllers, and view engines, making MVC available in your application.

You can pass data from a controller to a Razor view using multiple techniques, including ViewBag, ViewData, and TempData. These options allow you to share data between a controller and a view, but they have different lifetimes and use cases.

To extend the default user store in ASP.NET Core Identity, you would typically implement the IUserStore interface. This interface allows you to customize how user information is stored and managed. You can create a custom user store by implementing this interface and providing your own data storage mechanisms.

To prevent integration tests from impacting the actual database, an in-memory database is often used. It simulates database behavior but operates entirely in memory, making tests faster and isolated. It's a common practice in ASP.NET Core testing.