ASP.NET Core's modular architecture allows developers to include only the necessary dependencies they need for their application. This reduces the size and overhead of the application, making it more efficient and scalable. Developers can choose and add libraries and frameworks as per their project requirements.

In the project.json file, the "dependencies" section is where you list the packages that your ASP.NET Core project depends on. These packages can be libraries, frameworks, or other code components.

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.

To create a custom tag helper in ASP.NET Core, you need to create a class and decorate it with the TagHelper attribute. This attribute marks the class as a tag helper, allowing it to process and modify HTML tags in Razor views.

The "UserManager" in ASP.NET Core Identity is primarily responsible for managing user accounts. It provides a set of APIs for creating, updating, and deleting user accounts, as well as handling password-related operations, such as password reset and change. It is a fundamental component for user management in Identity.

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.