core-bundle/src/Resources/contao/controllers/BackendMain.php
public function run()
	{
		try
		{
			$version = PackageUtil::getVersion('contao/core-bundle');
		}
		catch (\OutOfBoundsException $e)
		{
			$version = PackageUtil::getVersion('contao/contao');
		}

		$this->Template = new BackendTemplate('be_main');
		$this->Template->version = $version;

		if (isset($GLOBALS['TL_LANG']['MSC']['version']))
		{
			$this->Template->version = $GLOBALS['TL_LANG']['MSC']['version'] . ' ' . $version;
		}

		$this->Template->main = '';

		// Ajax request
		if ($_POST && Environment::get('isAjaxRequest'))
		{
			$this->objAjax = new Ajax(Input::post('action'));
			$this->objAjax->executePreActions();
		}

		// Error
		if (Input::get('act') == 'error')
		{
			$this->Template->error = $GLOBALS['TL_LANG']['ERR']['general'];
			$this->Template->title = $GLOBALS['TL_LANG']['ERR']['general'];

			@trigger_error('Using act=error has been deprecated and will no longer work in Contao 5.0. Throw an exception instead.', E_USER_DEPRECATED);
		}
		// Welcome screen
		elseif (!Input::get('do') && !Input::get('act'))
		{
			$this->Template->main .= $this->welcomeScreen();
			$this->Template->title = $GLOBALS['TL_LANG']['MSC']['home'];
		}
		// Open a module
		elseif (Input::get('do'))
		{
			$picker = null;

			if (isset($_GET['picker']))
			{
				$picker = System::getContainer()->get('contao.picker.builder')->createFromData(Input::get('picker', true));

				if ($picker !== null)
				{
					if (($menu = $picker->getMenu()) && $menu->count() > 1)
					{
						$this->Template->pickerMenu = System::getContainer()->get('contao.menu.renderer')->render($menu);
					}
				}
			}

			$this->Template->main .= $this->getBackendModule(Input::get('do'), $picker);
			$this->Template->title = $this->Template->headline;
		}

		return $this->output();
	}
public function run()
	{
		try
		{
			$version = PackageUtil::getVersion('contao/core-bundle');
		}
		catch (\OutOfBoundsException $e)
		{
			$version = PackageUtil::getVersion('contao/contao');
		}

		$this->Template = new BackendTemplate('be_main');
		$this->Template->version = $version;

		if (isset($GLOBALS['TL_LANG']['MSC']['version']))
		{
			$this->Template->version = $GLOBALS['TL_LANG']['MSC']['version'] . ' ' . $version;
		}

		$this->Template->main = '';

		// Ajax request
		if ($_POST && Environment::get('isAjaxRequest'))
		{
			$this->objAjax = new Ajax(Input::post('action'));
			$this->objAjax->executePreActions();
		}

		// Error
		if (Input::get('act') == 'error')
		{
			$this->Template->error = $GLOBALS['TL_LANG']['ERR']['general'];
			$this->Template->title = $GLOBALS['TL_LANG']['ERR']['general'];

			@trigger_error('Using act=error has been deprecated and will no longer work in Contao 5.0. Throw an exception instead.', E_USER_DEPRECATED);
		}
		// Welcome screen
		elseif (!Input::get('do') && !Input::get('act'))
		{
			$this->Template->main .= $this->welcomeScreen();
			$this->Template->title = $GLOBALS['TL_LANG']['MSC']['home'];
		}
		// Open a module
		elseif (Input::get('do'))
		{
			$picker = null;

			if (isset($_GET['picker']))
			{
				$picker = System::getContainer()->get('contao.picker.builder')->createFromData(Input::get('picker', true));

				if ($picker !== null)
				{
					if (($menu = $picker->getMenu()) && $menu->count() > 1)
					{
						$this->Template->pickerMenu = System::getContainer()->get('contao.menu.renderer')->render($menu);
					}
				}
			}

			$this->Template->main .= $this->getBackendModule(Input::get('do'), $picker);
			$this->Template->title = $this->Template->headline;
		}

		return $this->output();
	}
Run the controller and parse the login template

@return Response