by default this file in end in the in the right place. If you want improve google page speed, need to do another changes. I have 6 years experience in cs cart cms and can you help with this.

If you really want to know where you can change this

1. templates/common/scripts.tpl tag {scripts}{/scripts} gather all js files and by hooks :)
2. here on backend drawning - ?432432423 - time param /public_html/app/functions/smarty_plugins/function.script.php

line - $src = Registry::get('config.current_location') . '/' . fn_link_attach($params['src'], 'ver=' . Tygh::$app['assets_cache_key']);

3. and 34 line you can add here - $scripts[$params['src']] = '<script' like this $scripts[$params['src']] = '<script defer async'