Built in function and it's pretty helpful to understand WP global variable

wp-load-template.php

<?php
function load_template( $_template_file, $require_once = true ) {
	global $posts, $post, $wp_did_header, $wp_query, $wp_rewrite, $wpdb, $wp_version, $wp, $id, $comment, $user_ID;

	if ( is_array( $wp_query->query_vars ) ) {
		extract( $wp_query->query_vars, EXTR_SKIP );
	}

	if ( isset( $s ) ) {
		$s = esc_attr( $s );
	}

	if ( $require_once ) {
		require_once( $_template_file );
	} else {
		require( $_template_file );
	}
}