Insert wordpress tags below posts via functions.php

If all you want to do is add tags to the post body without writing them into the theme templates, all you really need to do is:

add_filter(
  'the_content',
  function($content) {
    return $content.get_the_tag_list();
  }
);

See get_the_tag_list() for arguments. That, by the way, is basically what the the_tags() function does.

However, there are other ways to do it and those ways may be better depending upon you exact requirements. As written, your question is pretty light on detail.