core-bundle/src/Resources/contao/library/Contao/TemplateLoader.php
public static function initialize()
	{
		$objFilesystem = new Filesystem();
		$container = System::getContainer();
		$strCacheDir = $container->getParameter('kernel.cache_dir');

		// Try to load from cache
		if (file_exists($strCacheDir . '/contao/config/templates.php'))
		{
			self::addFiles(include $strCacheDir . '/contao/config/templates.php');
		}
		else
		{
			try
			{
				foreach (System::getContainer()->get('contao.resource_finder')->findIn('templates')->name('*.html5') as $file)
				{
					/** @var SplFileInfo $file */
					self::addFile($file->getBasename('.html5'), rtrim($objFilesystem->makePathRelative($file->getPath(), $container->getParameter('kernel.project_dir')), '/'));
				}
			}
			catch (\InvalidArgumentException $e) {}
		}
	}
public static function initialize()
	{
		$objFilesystem = new Filesystem();
		$container = System::getContainer();
		$strCacheDir = $container->getParameter('kernel.cache_dir');

		// Try to load from cache
		if (file_exists($strCacheDir . '/contao/config/templates.php'))
		{
			self::addFiles(include $strCacheDir . '/contao/config/templates.php');
		}
		else
		{
			try
			{
				foreach (System::getContainer()->get('contao.resource_finder')->findIn('templates')->name('*.html5') as $file)
				{
					/** @var SplFileInfo $file */
					self::addFile($file->getBasename('.html5'), rtrim($objFilesystem->makePathRelative($file->getPath(), $container->getParameter('kernel.project_dir')), '/'));
				}
			}
			catch (\InvalidArgumentException $e) {}
		}
	}
Find the templates in the Contao resource folders.