Startup.cs class in ASP.NET Core was changed and now it is clearer and divided in better sections. This is my opinion. The first thing that we have to understand: what is it for? How is it run? I begin the answer that this class is intended to be the initial point of project, by default the class called Startup is initialized by configuration of host, I am going to explain later. In this class is initialized all configurations needed to the project and now I am going to show so you can understand better.