A, Program.cs
1, the beginning of everything from Main, Main method is the entry contains the entire application
ASP.NET Core applications can configure and start the host (Host).
Host is responsible for application startup and lifecycle management, server configuration and request processing pipeline.
Logging the host can also be provided, and the configuration dependency injection.
The host also includes host Web host (IWebHostBuilder) and Universal Host (IHostBuilder).
According to the official website:
