One way to alter the page title is to use hook_preprocess_HOOK() and then set the new page title depending on the current route name.

function MY_MODULE_preprocess_page_title(&$variables) {
  $route_name = \Drupal::routeMatch()->getRouteName();
  if ($route_name === 'entity.user.edit_form') {
    $variables['title'] = 'Edit My Account';
  }
}

This will only change the title in the Page title block. To actually change the <title> HTML tag you can use the following code snippet:

function MY_MODULE_preprocess_html(&$variables) {
  $route_name = \Drupal::routeMatch()->getRouteName();
  if ($route_name === 'entity.user.edit_form') {
    $variables['head_title']['title'] = t('Edit My Account')
  }
}

If you want to hide a field inside the Inline Entity Form you have to use the hook_inline_entity_form_entity_form_alter() hook – standard form alter hook won’t work. For example, to hide the status field you can do something like this:

/**
 * Implements hook_inline_entity_form_entity_form_alter().
 */
function MY_MODULE_inline_entity_form_entity_form_alter(&$entity_form, &$form_state) {
  if (in_array($entity_form['#entity_type'], ['YOUR_ENTITY_TYPE_1', 'YOUR_ENTITY_TYPE_2'])) {
    $entity_form['status']['#access'] = FALSE;
  }
}

Hiding the Add to Cart button in Drupal Commerce 2.x is easy. Just alter the commerce_order_item_add_to_cart_form form, and set the disabled property of the button to true.

use Drupal\Core\Form\FormStateInterface;

function MY_MODULE_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if (strpos($form_id, 'commerce_order_item_add_to_cart_form') !== FALSE) {
    $product = $form_state->getFormObject()->getEntity()->getPurchasedEntity()->getProduct();
    
    $flag_service = \Drupal::service('flag');
    $flag = $flag_service->getFlagById('sold_out');
    $flagging = $flag_service->getFlagging($flag, $product);

    if ($flagging !== NULL ) {
      $form['actions']['submit']['#value'] = t('Out of stock');
      $form['actions']['submit']['#disabled'] = TRUE;
    }
  }
}

If you have only one payment method, then Drupal Commerce 2.x will hide it. We can change that behavior in the form alter hook.

use Drupal\Core\Form\FormStateInterface;

function MY_MODULE_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'commerce_checkout_flow_multistep_default') {
    if (isset($form['payment_information']['payment_method']['#access'])) {
      $form['payment_information']['payment_method']['#access'] = TRUE;
    }
  }
}

If you are using the UI you can add a Recaptcha only to the entire checkout form, and that usually is not what you want. Fortunately, adding a Recaptcha to a certain step is easy:

use Drupal\Core\Form\FormStateInterface;

function MY_MODULE_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'commerce_checkout_flow_multistep_default' && $form['#step_id'] == 'order_information') {
    $form['captcha'] = [
      '#type' => 'captcha',
      '#captcha_type' => 'recaptcha/reCAPTCHA',
    ];
  }
}

In some cases, you may want to add a role when a new user is created regardless of how the user is created (UI, GraphQL, Rest API, or something else). The hook_ENTITY_TYPE_presave() hook is perfect for this.

use Drupal\user\UserInterface;

function MY_MODULE_user_presave(UserInterface $user) {
  if ($user->isNew() && !$user->hasRole('frontend_app')) {
    $user->addRole('frontend_app');
  }
}

Adding a menu link to some existing menu programmatically (for example in an update hook) is something I have to do fairly often. Here's how you can add a new menu link to the main menu:

use Drupal\menu_link_content\Entity\MenuLinkContent;

function MY_MODULE_update_8001() {
  MenuLinkContent::create([
    'title' => 'About',
    'link' => ['uri' => 'internal:/about'],
    'menu_name' => 'main',
    'weight' => -50,
  ])->save();
}

And if you want to add a submenu link you first have to load the parent link and then create a submenu link:

use Drupal\menu_link_content\Entity\MenuLinkContent;

function MY_MODULE_update_8001() {
  $menu_link_parents = \Drupal::entityTypeManager()
    ->getStorage('menu_link_content')
    ->loadByProperties([
      'title' => 'Dashboard',
      'menu_name' => 'main',
    ]);

  $menu_link_parent = reset($menu_link_parents);

  if ($menu_link_parent) {
    MenuLinkContent::create([
      'title' => 'Add tasks',
      'link' => ['uri' => 'internal:/add-task'],
      'menu_name' => 'main',
      'weight' => -50,
      'parent' => $menu_link_parent->getPluginId(),
    ])->save();
  }
}

Rendering a menu in a form requires a little bit more lines of code than rendering a View, but it is also very easy thanks to the menu.link_tree service. In the following example, we are rendering the main menu.

use Drupal\Core\Menu\MenuTreeParameters;

$menu_parameters = new MenuTreeParameters();
$manipulators = [
  ['callable' => 'menu.default_tree_manipulators:checkNodeAccess'],
  ['callable' => 'menu.default_tree_manipulators:checkAccess'],
  ['callable' => 'menu.default_tree_manipulators:generateIndexAndSort'],
];
$tree = \Drupal::menuTree()->load('main', $menu_parameters);
$tree = \Drupal::menuTree()->transform($tree, $manipulators);

$form['content'] = \Drupal::menuTree()->build($tree);

Rendering Views in a form is super easy thanks to the buildRenderable() method, which builds the render array for the given display. To render a View, you just need to know the machine name of the View and display ID (usually page_1 or block_1). To render the Content View do this:

use Drupal\views\Views;
$view = Views::getView('content');
$form['content'] = $view->buildRenderable('page_1');

To get the field definitions of any entity type use the following code. We first get all bundles, and then for each bundle we are getting the field definitions.

$bundles = \Drupal::service('entity_type.bundle.info')->getBundleInfo('node');

foreach ($bundles as $bundle => $data) {
  $fields = \Drupal::service('entity_field.manager')->getFieldDefinitions('node', $bundle);
}