default.aspxdefault.aspx.csdefault.axpx.designer.cs

A solution is a collection of projects, I can click on solution and add
one more project to it.
A solution has sln extension
where as the project has extension of .csproj

Webforms have extension of aspx.
you can bring up toolbox to add buttons or panels, this option is in
view menu

properties windows is used to change properties of webpage content , we
can change the properties of button like names font or anything, if it
is not available you can bring it up from view menu

Asp .net creates a a default webform automatically, its extension is
.aspx. so a webform has three modes, Design Mode (where you can change
or edit in design mode ) or source mode  or you can see the split
between design and source mode

A webform has a code behind it , you can bring up the code by right
clicking in anywhere in defaut.aspx and selecting "show code option" It
opens up default.aspx.cs file

NOTE: to bring up or to add code of the Button or any element of tool
box in default.aspx.cs file, you should double click the element, in
this case, the button, in Design mode of Default.aspx. This would add a
method related to the button in Default.aspx.cs where you can add the
functionality to the button

Default.aspx.cs (code behind file), This contains the code that user
writes, forexample on the page load what should happen, or what would
happen if button "click" is clicked.

Default.aspx.designer.cs: when we drag and drop button in default.aspx,
the auto generated code is added to Default.aspx.designer.cs . You
should not change the code in the designer file, because that code might
later be modified by c#.

in Default.aspx we have a line
<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.Master"
AutoEventWireup="true" CodeBehind="Default.aspx.cs"
Inherits="WebApplication1._Default" %>
which is called the page directive and the attribute CodeBehind tells
the what code it related to the file
A webform is associated with its codefile using @pagedirective
which means every page or webform we create will have the code
associated with it and all the files will have corresponding page
directive.

A webform's HTML can be edited either in Source or Design mode. You can
choose split mode, which shows both the DEsign and the Sources at the
same time.