Folder patterns

To load pattern php files, create a patterns folder, add a file.php with at least:

<?php

/*
 * Title: required
 * Slug: required
 */

Some content

Having a php file can be handy for translations.

Integration in a template file example :

<!-- wp:pattern {"slug":"df/content-404"} /-->

Note: for WordPress to be able to automatically look into the patterns folder, one need to add in wp-config.php:

define( 'WP_ENVIRONMENT_TYPE', 'development' ); 
define( 'WP_DEVELOPMENT_MODE', 'theme' );

Sources: