Hooking custom content after the comments section

You should be able to use this hook: https://docs.generatepress.com/article/generate_after_main_content/

Let me know if you need help with the function/conditional :)

I was able to insert the code to the right place, but I still need to make it conditional so that the code is shown only on single posts.

Something like this should do it:

add_action( 'generate_after_main_content','tu_after_main_content' );  
function tu_after_main_content() { 
    if ( ! is_single() ) { ?> 
        Your Youtube video here
    <?php }
}

Adding PHP: https://docs.generatepress.com/article/adding-php/