Add shortcode inside of the_content()

by

You can use the_content Hook: https://developer.wordpress.org/reference/hooks/the_content/

add_filter( 'the_content', 'my_shortchode_in_single_page' );
function my_shortchode_in_single_page($content){
    if(is_single())
        return $content . do_shortcode('YOUR SHORTCODE HERE');

    return $content;
}