WordPress Block Editor: Define a block template that includes a reusable block

gutenberg.php

<?php

/**
 * Define a block template for a post type that includes a reusable block.
 * Find the reusable block ID at /wp-admin/edit.php?post_type=wp_block
 *
 * @return void
 */
add_action('init', function () {
    $post_type_object = get_post_type_object('page'); // Set the post type
    $post_type_object->template = [
        ['core/paragraph'],
        ['core/block', ['ref' => 48170]], // Reusable block ID
    ];
});