How to add a default predefined thumbnail when creating a brand new post?

I found an adequate solution.

Generally, I used the save_post_{$post->post_type} hook which gave me a $post_ID and a $post object.

I was just checking to see if there was a thumbnail and if is not there to add it. That is all.

add_action( 'save_post_book', function ($post_ID, $post){
    if ( get_post_meta($post_ID, '_thumbnail_id', true) ) :
        return;
    endif;
    
    if($thumbnail_id = get_option('my_default_thumbnail_id', false)) {
        add_post_meta($post_ID, '_thumbnail_id', $thumbnail_id);
    }
}, 10, 2);