How to build a background project with C# in VS? Is it like this after construction? How is the three-tier architecture established?

There are many definitions of background. Server script (C# page code) can also be said to be the background (as opposed to front-end scripts such as javascript), business processing logic can also be said to be the background, and database operations and the database itself are also the background.

Of course, the background in many scenarios refers to the business processing logic and related operations of the database.

The three layers of WEB include MVC, page, business logic and database operation.

You have created a WEB project, which is nothing. A complex application may have dozens or even dozens of various types of projects, such as your WEB service project, WCF service project, class library project, test project and so on.