ASPX files usually will have the UI and will which is usually HTML tags, some ASP.NET server control embed code (which ultimately produce some HTML markups). ASPX.CS file (usually called "code-behind") will have server-side coding in C#.

If needed, I would relate ASPX page to View and ASPX.CS to Controller action methods.

You have to remember that in webforms, there are ASP.NET controls we will be using in the ASPX file to render some HTML. Examples are TextBox, DataGrid, etc. In MVC, there is nothing called Server control. The View will be pure,