is_404() and is_page() is simply just conditionals which just checks if a condition exists. Like is_404() simply checks if the current page is a 404 page and returns true on success and false on failure.
VERY IMPORTANT NOTICE: Conditional query tags do not work before the query is run. Before then, they always return false
You can get objects from the current page by just simply doing a var_dump on get_queried_object() like for example
<?php
$queried_object = get_queried_object();
var_dump( $queried_object );
?>
EDIT
All of these conditionals are set in the WP_Query class, and then separately wrapped in a function in wp-includes/query.php. For instance, is_404() is just a wrapper for $wp_query->is_404();
697 /**
698 * Is the query a 404 (returns no results)?
699 *
700 * @see WP_Query::is_404()
701 * @since 1.5.0
702 * @uses $wp_query
703 *
704 * @return bool
705 */
706 function is_404() {
707 global $wp_query;
708
709 if ( ! isset( $wp_query ) ) {
710 _doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '3.1' );
711 return false;
712 }
713
714 return $wp_query->is_404();
715 }