How to call function at the bottom of post using plugin?

If you currently are using shortcodes, then most probably you want to display some dynamic content at the bottom of the post. If so, then there is a hook that will allow you to do that: the_content

If you want to modify the content only when single post is displayed, then you can use this code:

function my_the_content_filter( $content ) {
    if ( is_single() && get_the_ID() === get_queried_object_id() ) {
        $content .= '<div>Some additional content or whatever you want to be printed in here...</div>';
    }

    return $content;
}
add_filter( 'the_content', 'my_the_content_filter' );

If you want to modify it everywhere, then you should change the condition to match your needs.