Sample plugin for redirecting a WordPress site to a specific URL.

wp-site-redirect.php

<?php

/*
 * Plugin Name:
 * Plugin URI:
 * Description:
 * Version: 1.0
 * Author:
 * Author URI:
 * License: GPL2
 * License URI: https://www.gnu.org/licenses/gpl-2.0.html
 */

add_action(
	'init',
	function () {
		global $pagenow;
		// Allow requests to /wp-admin and wp-login so that admins can attempt to login
		if ( 'wp-login.php' !== $pagenow && ! is_admin() ) {
			wp_redirect( 'https://mynewsite.com', 301 );
			exit;
		}
	}
);