You have to look into OnLoad method where are added all admin pages :

protected override void OnLoad(EventArgs e)
    {
      AssemblyInformationalVersionAttribute customAttribute = Assembly.GetAssembly(typeof (Default)).GetCustomAttributes(typeof (AssemblyInformationalVersionAttribute), false)[0] as AssemblyInformationalVersionAttribute;
      Assert.IsNotNull((object) customAttribute, "assemblyInfoVersion is null");
      this.Version.Text = customAttribute.InformationalVersion;
      List<Default.PageDescription> pageDescriptionList = new List<Default.PageDescription>()
      {
        new Default.PageDescription()
        {
          Name = "Event Queue Statistics",
          Description = "An overview of the Event Queue" + (DataProviderHelper.IsOracleEnabled ? ".<br/><i style=\"color: red;\">This page is not supported for Oracle provider.</i>" : string.Empty),
          Link = "/sitecore/admin/EventQueueStats.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Database Cleanup",
          Description = "Perform various cleanup operations on specific databases",
          Link = "/sitecore/admin/DBCleanup.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Jobs Viewer",
          Description = "An overview of the jobs",
          Link = "/sitecore/admin/Jobs.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Package Item",
          Description = "Make a package of items and their dependencies",
          Link = "/sitecore/admin/PackageItem.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Publish Queue Statistics",
          Description = "An overview of the Publish Queue",
          Link = "/sitecore/admin/PublishQueueStats.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Raw Search",
          Description = "Search for the specific string in a database or in the file system" + (DataProviderHelper.IsOracleEnabled ? ".<br/><i style=\"color: red;\">The database search is not supported for Oracle provider.</i>" : string.Empty),
          Link = "/sitecore/admin/RawSearch.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Rebuild Reporting Database",
          Description = "Rebuild the Reporting database",
          Link = "/sitecore/admin/RebuildReportingDB.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Security Tools",
          Description = "Login and user management tools",
          Link = "/sitecore/admin/SecurityTools.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Sql Shell",
          Description = "Use connection strings to execute SQL scripts" + (DataProviderHelper.IsOracleEnabled ? ".<br/><i style=\"color: red;\">This page is not supported for Oracle provider.</i>" : string.Empty),
          Link = "/sitecore/admin/SqlShell.aspx"
        },
        new Default.PageDescription()
        {
          Name = "User Info",
          Description = "Logged in user details",
          Link = "/sitecore/admin/UserInfo.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Cache",
          Description = "An overview of the caches",
          Link = "/sitecore/admin/cache.aspx"
        },
        new Default.PageDescription()
        {
          Name = "DB Browser",
          Description = "The interface for various item manipulations",
          Link = "/sitecore/admin/dbbrowser.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Install Language",
          Description = "Install a new content language",
          Link = "/sitecore/admin/InstallLanguage.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Pipeline Profiler",
          Description = "An overview of pipeline execution",
          Link = "/sitecore/admin/pipelines.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Remove Broken Links",
          Description = "Remove broken links from a database",
          Link = "/sitecore/admin/RemoveBrokenLinks.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Restore Item",
          Description = "Restore items from the archive",
          Link = "/sitecore/admin/restore.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Serialization",
          Description = "Serialize and revert a database",
          Link = "/sitecore/admin/serialization.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Set Sitecore Application Center Endpoint",
          Description = "Change the endpoint address of the App Center",
          Link = "/sitecore/admin/SetSACEndpoint.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Show Config",
          Description = "Merge configuration files",
          Link = "/sitecore/admin/ShowConfig.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Show Config Layers",
          Description = "Merge configuration files depending on configuration layers and roles",
          Link = "/sitecore/admin/ShowConfigLayers.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Dependency Injection Configuration",
          Description = "Check the configured services",
          Link = "/sitecore/admin/ShowServicesConfig.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Rendering Statistics",
          Description = "An overview of the performance of renderings",
          Link = "/sitecore/admin/stats.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Unlock Admin",
          Description = "Unlock Admin user",
          Link = "/sitecore/admin/unlock_admin.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Update Installation Wizard",
          Description = "Install Sitecore updates",
          Link = "/sitecore/admin/UpdateInstallationWizard.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Logs",
          Description = "View and download Sitecore log files",
          Link = "/sitecore/admin/Logs.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Media Hash Generator",
          Description = "Generate hash values for dynamic image scaling URLs",
          Link = "/sitecore/admin/MediaHash.aspx"
        },
        new Default.PageDescription()
        {
          Name = "Support Package",
          Description = "Generate a package with support information",
          Link = "/sitecore/admin/SupportPackage.aspx"
        }
      };
      Repeater pagesRepeater = this.PagesRepeater;
      List<Default.PageDescription> source = pageDescriptionList;
      StringComparer ordinalIgnoreCase = StringComparer.OrdinalIgnoreCase;
      IOrderedEnumerable<Default.PageDescription> orderedEnumerable = source.OrderBy<Default.PageDescription, string>((Func<Default.PageDescription, string>) (p => p.Name), (IComparer<string>) ordinalIgnoreCase);
      pagesRepeater.DataSource = (object) orderedEnumerable;
      this.PagesRepeater.DataBind();
    }