The primary difference between the "SDK" (Software Development Kit) and "Runtime" is that the SDK includes tools for development, such as compilers, libraries, and other utilities needed to create applications, while the Runtime includes only what is necessary to run applications built with the SDK. This separation allows developers to build applications on one system (using the SDK) and run them on another (using the Runtime) without the need for development tools.

ASP.NET Core is designed to be a high-performance and scalable framework. It's optimized for building fast and efficient web applications, making it suitable for modern, demanding web development scenarios. Unlike some proprietary frameworks, ASP.NET Core is open-source and cross-platform, which aligns with the industry's move towards open standards and flexibility.

To use tag helpers in a Razor view, you need to add the "@using" directive at the beginning of your view file. This directive imports the necessary namespaces and makes the tag helpers available for use in the view.

To return a custom error view when handling errors in your ASP.NET Core MVC application, you should use the ViewResult action result. You can specify the view to be rendered, providing a custom error page to enhance the user experience when errors occur.

The CreateUserAsync method of the UserManager class is primarily used to create a new user in ASP.NET Core Identity. This method handles user creation and automatically generates and stores the necessary security information.

For ensuring that your data access layer correctly interacts with your business logic layer in a multi-tier application, "Integration Testing" is the most suitable approach. Integration tests verify the interactions between different components or tiers of an application, ensuring that they work together as intended.

ASP.NET Core is known for its modularity, which enables developers to build applications with only the necessary libraries and components. This modularity reduces complexity and avoids the inclusion of unnecessary legacy code, promoting flexibility in application development.

The @addTagHelper directive is used in the _ViewImports.cshtml file to ensure that tag helpers are available across all Razor views in an ASP.NET Core application. This directive registers tag helpers, allowing you to use them throughout your views.

Entity Framework Core (EF Core) is the Microsoft library that allows you to interact with databases in an ASP.NET Core project without the need to write extensive SQL code. It provides a high-level, object-oriented approach to database operations.

In an ASP.NET Core project, you typically define the default layout for Razor views in the "_ViewImports.cshtml" file. This file allows you to specify common directives and layout settings that should apply to multiple views in your project, simplifying the process of maintaining a consistent layout across your application.