customer-riverside/web/modules/custom/riverside_pt/riverside_pt.module

<?php

use Drupal\Core\Breadcrumb\Breadcrumb;
use Drupal\Core\Link;
use Drupal\Core\Routing\RouteMatchInterface;

function riverside_pt_page_attachments(array &$attachments): void {
  $attachments['#attached']['library'][] = 'riverside_pt/app';
}

function riverside_pt_theme(): array {
  return [
    'riverside_pt_header' => [
      'variables' => [
        'site_name'    => NULL,
        'home_url'     => NULL,
        'menu_items'   => [],
        'current_path' => NULL,
      ],
    ],
    'riverside_pt_home' => [
      'variables' => [],
    ],
  ];
}

function riverside_pt_page_top(array &$page_top): void {
  $page_top['rpt_header'] = [
    '#theme' => 'riverside_pt_header',
    '#cache' => ['contexts' => ['url.path']],
  ];
}

function riverside_pt_preprocess_riverside_pt_header(array &$variables): void {
  $variables['site_name'] = \Drupal::config('system.site')->get('name');
  $variables['home_url']  = \Drupal\Core\Url::fromRoute('<front>')->toString();

  $tree_service = \Drupal::service('menu.link_tree');
  $params = new \Drupal\Core\Menu\MenuTreeParameters();
  $params->setMaxDepth(1);
  $tree = $tree_service->load('main', $params);
  $tree = $tree_service->transform($tree, [
    ['callable' => 'menu.default_tree_manipulators:checkAccess'],
    ['callable' => 'menu.default_tree_manipulators:generateIndexAndSort'],
  ]);

  $seen  = [];
  $items = [];
  foreach ($tree as $element) {
    if (!$element->access->isAllowed()) {
      continue;
    }
    $url = $element->link->getUrlObject()->toString();
    if (in_array($url, $seen, TRUE)) {
      continue;
    }
    $seen[]  = $url;
    $title   = (string) $element->link->getTitle();
    $items[] = [
      'title'  => $title,
      'url'    => $url,
      'is_cta' => ($title === 'Book An Appointment' || $title === 'Contact'),
    ];
  }
  $variables['menu_items']   = $items;
  $variables['current_path'] = \Drupal::request()->getPathInfo();
}

function riverside_pt_system_breadcrumb_alter(Breadcrumb &$breadcrumb, RouteMatchInterface $route_match, array $context): void {
  if ($route_match->getRouteName() === 'riverside_pt.booking') {
    $breadcrumb = new Breadcrumb();
    $breadcrumb->addLink(Link::createFromRoute('← Back', 'riverside_pt.schedule'));
    $breadcrumb->addCacheContexts(['route']);
  }
}

function riverside_pt_mail(string $key, array &$message, array $params): void {
  if ($key !== 'booking_request') {
    return;
  }

  $start = new \DateTime($params['start']);
  $end   = new \DateTime($params['end']);

  $message['subject'] = 'Booking request — ' . $start->format('M j, Y g:i A');
  $lines = [
    'Name:  ' . $params['first_name'] . ' ' . $params['last_name'],
    'Phone: ' . $params['phone'],
    'Slot:  ' . $start->format('l, F j, Y') . ', ' . $start->format('g:i A') . '–' . $end->format('g:i A'),
  ];

  if (!empty($params['comments'])) {
    $lines[] = 'Comments: ' . $params['comments'];
  }

  $message['body'][] = implode("\n", $lines);
}