Template::mergeTemplatePaths() 

public function mergeTemplatePaths ( $defaultTemplatePaths = [], $additionalTemplatePaths = [] ) {
	$pathsToMerge = ['templateRootPaths', 'partialRootPaths', 'layoutRootPaths'];
	foreach ($pathsToMerge as $field) {
		if ($paths = $additionalTemplatePaths[$field] ?? false) {
			if (!isset($defaultTemplatePaths[$field])) {
				$defaultTemplatePaths[$field] = [];
			}
			ArrayUtility::mergeRecursiveWithOverrule($defaultTemplatePaths[$field], $paths);
		}
	}
	if ($path = $additionalTemplatePaths['template'] ?? false) {
		$defaultTemplatePaths['template'] = $path;
	}
	return $defaultTemplatePaths;
}