If your posts page is a static page selected under Settings > Reading > Front page displays, then get_queried_object_id()
will return the ID for that page which you can use to fetch meta data.
$check_meta = get_post_meta( get_queried_object_id(), 'checkbox', true );
$header_meta = get_post_meta( get_queried_object_id(), 'header', true );
$textarea_meta = get_post_meta( get_queried_object_id(), 'textarea', true );