To add a preloader in Wordpress, first copy the code of the second step in the functions.php file, then replace the link of your desired animation loader with the default loader and enter the duration of the loader display, and save the file.
To to create Custom login page in WordPress, first copy one of the following three templates in the functions.php file. Then replace the default background image and the logo with your own and save the file.
To redirect pages in WordPress without plugins, first copy the code of step 2 in the functions.php file, then change the "from" and "to" URLs to your desired URLs, and save the file.
To create a child theme, first find the "Text Domain" of your current theme in the stylesheet file. Then create the three files described in step two, zip them, and upload them to the site's themes section.