Migrate from prefixing convention to PHP namespacing

namespace-2.php

<?php
/**
 * Description
 *
 * @package     ${NAMESPACE}
 * @since       1.0.0
 * @author      hellofromTonya
 * @link        https://KnowTheCode.io
 * @license     GNU-2.0+
 */
namespace KnowTheCode\Root;
// aliasing
use KnowTheCode\InsOutsPHPNamespacing\Sandbox as sandbox;
// import a function using PHP 5.6 and up
use function KnowTheCode\InsOutsPHPNamespacing\Sandbox\{get_the_ID, get_post_id};
function get_the_other_ID() {
	ddd( 'yup it worked');
}
//Sandbox\get_the_ID();
\KnowTheCode\InsOutsPHPNamespacing\Sandbox\get_post_id();
sandbox\get_post_id();
get_post_id();
//\get_the_ID();

namespace.php

<?php
/**
 * Description
 *
 * @package     ${NAMESPACE}
 * @since       1.0.0
 * @author      hellofromTonya
 * @link        https://KnowTheCode.io
 * @license     GNU-2.0+
 */
namespace KnowTheCode\Root;
// aliasing
use KnowTheCode\InsOutsPHPNamespacing\Sandbox as sandbox;
// import a function using PHP 5.6 and up
use function KnowTheCode\InsOutsPHPNamespacing\Sandbox\get_the_ID;
use function KnowTheCode\InsOutsPHPNamespacing\Sandbox\get_post_id;
function get_the_other_ID() {
	ddd( 'yup it worked');
}
// relative naming
Sandbox\get_the_ID();
// fully qualified name
\KnowTheCode\InsOutsPHPNamespacing\Sandbox\get_the_ID();
// alias naming
sandbox\get_the_ID();
// importing with PHP 5.6 and up
get_the_ID();
// this is one out of WordPress Core.
//\get_the_ID();

WP_Query is not namespaced. Therefore you have two choices:

1. You can use the fully qualified class name in the code like this:

$query = new \WP_Query( $args );

2. Import first to avoid having to use the backslash namespace separator:

use WP_Query

Then the query code is this:

$query = new WP_Query( $args );