How does .NET framework deal with app.config?

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C#

XML

.NET

Now, many project depend on configure file, .net framework provide lots of useful class to read it.but i don't know the real process when read the specific elements(add) from app.config：
eg:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <configSections>
  </configSections>
  <connectionStrings>
     <add name="LightCtrlSysLib.Properties.Settings.LightDbConnectionString"
      connectionString="Data Source=(local);Initial Catalog=LightDb;
                      Integrated Security=True"
      providerName="System.Data.SqlClient" />
  </connectionStrings>
</configuration>

is it will trigger specific event?I know my idea is so simple,if wrong.

how to design xml file realize this idea?