Razor Layout Views in ASP.NET Core are primarily used to define the common structure or layout that multiple Razor views within your application will share. This allows you to maintain a consistent look and feel across various parts of your web application. They help separate the presentation logic from the content-specific views.

To create a new ASP.NET Core MVC project, you would typically use an integrated development environment (IDE) like 'Visual Studio Code.' Visual Studio Code provides excellent support for ASP.NET Core development, including project templates and extensions that streamline the development process.

The project.json file was used in the early versions of ASP.NET Core (then known as ASP.NET 5), but it was replaced with the .csproj file format in .NET Core 2.0 and beyond. The project.json file defined project dependencies and configuration settings.

Custom middlewares in ASP.NET Core are created using a delegate with the signature Func. This delegate takes an HttpContext as input and returns a Task, allowing you to write custom logic to handle requests and responses in the pipeline.

To specify multiple roles using the [Authorize] attribute, you can separate them with commas inside the Roles parameter, like this: [Authorize(Roles = "Admin, Manager")]. This allows access to the action or controller for users who belong to either the "Admin" role or the "Manager" role or both.

One of the core features of ASP.NET Core Identity is its ability to provide two-factor authentication (2FA). This adds an extra layer of security by requiring users to provide two forms of identification, typically something they know (password) and something they have (e.g., a mobile app-generated code) when logging in.

In the MVC (Model-View-Controller) design pattern, the Controller is primarily responsible for handling user input and interactions. It receives user requests, processes them, interacts with the Model to retrieve or manipulate data, and determines the appropriate View to render as a response.

The Identity middleware in ASP.NET Core primarily handles authentication. It intercepts requests to determine if a user is authenticated and provides features like cookie-based authentication, token-based authentication, and integration with external identity providers (e.g., Google, Facebook) for user login.

The primary language used for coding in ASP.NET Core is C#. While ASP.NET Core supports multiple languages, C# is the most commonly used language for building ASP.NET Core applications due to its strong integration with the framework and extensive tooling support.

To restrict access to registered users only, you would use the [Authorize] attribute. This attribute can be applied at the controller or action level and ensures that only authenticated users can access the specified resources. It doesn't require specifying roles, as it already implies authenticated users. [AllowAnonymous] would allow both anonymous and registered users, while [Authorize(Roles = "Admin")] and [Authorize(Roles = "User")] are role-based authorizations and are not suitable for this scenario.