How to access the full html of a drupal basic page?
You will need to edit your themes tpl files to make changes to those areas.  If you are adding a js file you can add it in your themes template.php file. I would recommend you read through this link for

Drupal 7:

https://www.drupal.org/docs/7/theming

Drupal 8:

https://www.drupal.org/docs/8/theming