drupal/core/modules/layout_discovery/layout_discovery.module

<?php

/**
 * @file
 * Provides hook implementations for Layout Discovery.
 */

use Drupal\Core\Render\Element;
use Drupal\Core\Template\Attribute;

/**
 * Implements hook_help().
 */
function layout_discovery_help($route_name) {
  switch ($route_name) {
    case 'help.page.layout_discovery':
      $output = '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('Layout Discovery allows modules or themes to register layouts, and for other modules to list the available layouts and render them.') . '</p>';
      $output .= '<p>' . t('For more information, see the <a href=":layout-discovery-documentation">online documentation for the Layout Discovery module</a>.', [':layout-discovery-documentation' => 'https://www.drupal.org/docs/8/api/layout-api']) . '</p>';
      return $output;
  }
}

/**
 * Implements hook_theme().
 */
function layout_discovery_theme() {
  return \Drupal::service('plugin.manager.core.layout')->getThemeImplementations();
}

/**
 * Prepares variables for layout templates.
 *
 * @param array &$variables
 *   An associative array containing:
 *   - content: An associative array containing the properties of the element.
 *     Properties used: #settings, #layout, #in_preview.
 */
function template_preprocess_layout(&$variables) {
  $variables['settings'] = $variables['content']['#settings'] ?? [];
  $variables['layout'] = $variables['content']['#layout'] ?? [];
  $variables['in_preview'] = $variables['content']['#in_preview'] ?? FALSE;

  // Create an attributes variable for each region.
  foreach (Element::children($variables['content']) as $name) {
    if (!isset($variables['content'][$name]['#attributes'])) {
      $variables['content'][$name]['#attributes'] = [];
    }
    $variables['region_attributes'][$name] = new Attribute($variables['content'][$name]['#attributes']);
  }
}