What is the diference between Post Metadata and Queried Object

Q1 – Is there a difference between these 2? – Post Metadata and Queried Object.

Yes.

And I won’t go into a detailed explanation (e.g. what a post metadata is), but these should help you for what you’re trying to do:

1. Post metadata are saved in the wp_postmeta table. (Note though, the table prefix might be different in your case, but wp_ is the default.)

2. The queried object in question refers to the post object/data which is retrieved from the wp_posts table, which contains fields like ID, post_type and comment_status — the one you’re trying to change.

So if you want to set the comment status of a post, there’s no need to add a custom metadata to the post. Just use wp_update_post().

Q2 – How do I set the default comment status for sfwd-essays post types to open ?

Not sure if there’s a specific setting for that during post type registration, but there’s a hook you can use: get_default_comment_status. For example for your post type:

add_filter( 'get_default_comment_status', function ( $status, $post_type ) {
    // Set to open if post type = sfwd-essays
    return ( 'sfwd-essays' === $post_type ? 'open' : $status );
}, 10, 2 );