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.

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.

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.

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.

Route constraints enable you to define restrictions on the data that can be matched by a route parameter. This restriction is usually based on the type of data expected for the parameter. For example, {id:int} enforces that the id parameter must be an integer type.

The "Authorize" attribute in ASP.NET Core Identity is commonly used to protect actions and controllers from unauthorized access. When applied to a controller or action, it requires that the user must be authenticated and authorized to access that resource.

In ASP.NET Core MVC, the ViewResult is typically used to return HTML content to the browser. It represents a view that should be rendered to generate the HTML response sent to the client.

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.

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.

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.

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.

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.