Thanks to the hook_user_format_name_alter() hook altering the username that is displayed for a user is really easy. Let's say that you have the First and Last name fields on the user entity. You can do something like this to alter the username:

use Drupal\Core\Session\AccountInterface;
use Drupal\user\Entity\User;

/**
 * Implements hook_user_format_name_alter().
 */
function MY_MODULE_user_format_name_alter(&$name, AccountInterface $account) {
  $full_name = [];

  $account = User::load($account->id());
  $first_name = $account->get('field_first_name')->value;
  $last_name = $account->get('field_last_name')->value;

  if (!empty($first_name)) {
    $full_name[] = $first_name;
  }

  if (!empty($last_name)) {
    $full_name[] = $last_name;
  }

  if (!empty($full_name)) {
    $name = implode(' ', $full_name);
  }
}

If you now try to get the username by using this $account->getDisplayName() you will see the new username that consists of the first and last name.

If you want to alter the title of the Read more link or perhaps add some CSS classes, you can do that by using the hook_node_links_alter() hook.

use Drupal\node\NodeInterface;

function MY_MODULE_node_links_alter(array &$links, NodeInterface $entity, array &$context) {
  if (!empty($links['node']['#links']['node-readmore']['title'])) {
    $links['node']['#links']['node-readmore']['title'] = t('View Details');
    $links['node']['#links']['node-readmore']['attributes']['class'][] = 'view-details-link';
  }
}

You can do this just for some content types, by checking the bundle value: if ($entity->bundle() == 'article') {}

Creating a modal with a title and some content in Ajax callback is easy:

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\OpenModalDialogCommand;

public function ajaxCallback(array $form, FormStateInterface $form_state) {
  $response = new AjaxResponse();
  $response->addCommand(new OpenModalDialogCommand($this->t('Title!'), $this->t('Some content.')));
  return $response;
}

If you also want to display a button in the modal, just provide the third parameter like this:

$options = [
  'buttons' => [
    'button1' => [
      'text' => $this->t('Close'),
      'id' => 'close-button',
      'onclick' => "jQuery('.ui-icon-closethick').click()",
    ],
  ],
];

...
$response->addCommand(new OpenModalDialogCommand($this->t('Title!'), $this->t('Some content.'), $options));
...

As you can see, by clicking on this button the modal will be closed.

If you want to programmatically check the Pathauto's Generate automatic URL alias checkbox you can do something like this:

use Drupal\pathauto\PathautoState;

$entity_type = 'node';
$entity_storage = \Drupal::entityTypeManager()->getStorage($entity_type);
$nodes = $entity_storage->loadMultiple();

foreach($nodes as $node) {
  $node->path->pathauto = PathautoState::CREATE;
  $node->save();
}

For terms just change the entity type to this:

$entity_type = 'taxonomy_term';

To uncheck the field instead of CREATE use SKIP constant:

$node->path->pathauto = PathautoState::SKIP;

Programatically rendering entity form in Drupal 8 and 9 is easy, provided that you want to render it using the default form mode.

// Load existing node
$node = \Drupal\node\Entity\Node::load(1);
// or create a new node
$node = \Drupal::entityTypeManager()->getStorage('node')->create(['type' => 'article']);

$form = \Drupal::service('entity.form_builder')->getForm($node);

If you want to render it using any other form mode like this for example:

$node = \Drupal\node\Entity\Node::load(1);
$form = \Drupal::service('entity.form_builder')->getForm($node, 'compact');

you will have to alter the entity definition and add your own form handler. Otherwise, you'll get the following error:

Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException: The "node" entity type did not specify a "compact" form class.

To add your own form class you can use the hook_entity_type_alter hook like this:

/**
 * Implements hook_entity_type_alter().
 *
 * Alters the entity definition and adds our own form handlers.
 */
function MY_MODULE_entity_type_alter(array &$entity_types) {
  $form_modes = \Drupal::service('entity_display.repository')
    ->getAllFormModes();

  foreach ($form_modes as $entity_type => $display_modes) {
    if ($entity_type !== 'node') {
      continue;
    }

    $type = $entity_types[$entity_type];
    foreach ($display_modes as $machine_name => $form_display) {
      if (isset($type->getHandlerClasses()['form']['default'])) {
        $default_handler_class = $type->getHandlerClasses()['form']['default'];
        $type->setFormClass($machine_name, $default_handler_class);
      }
    }
  }
}

To get a list of all field names for the fields whose field type is for example image you can use the entity_field.manager service:

$field_map = \Drupal::service('entity_field.manager')->getFieldMapByFieldType('image');

The field_map array will contain a list of fields by entity type.

Changing the order state in Drupal Commerce 2.x like this:

$order->set('state', 'canceled');
$order->save();

is not a good idea because you are not dispatching transition events. And some modules depend on those events. The proper way to change the order state is to apply state transition like this:

$order = \Drupal::entityTypeManager()
  ->getStorage('commerce_order')
  ->load(1);
$order->getState()->applyTransitionById('cancel');
$order->save();

You can find the list of transition IDs in the workflows file.

Sometimes you have to add a new language in Drupal 8/9 programmatically. For example, maybe you need to write an update hook that will add a new language. This is how you can do it:

use Drupal\language\Entity\ConfigurableLanguage;

$language = ConfigurableLanguage::createFromLangcode('sr');
$language->save();

If you don't know the langcode, you can find it by inspecting the select box on the /admin/config/regional/language/add page. 

Price in Drupal Commerce 2.x is not a number but a class. This class has a price number and currency code properties, and both are strings. So, to set the product price programmatically you have to first instantiate the Price class:

$price = new \Drupal\commerce_price\Price('9.99', 'EUR');

and then use created Price object to set the product price:

$product_variation = \Drupal\commerce_product\Entity\ProductVariation::load(1);
$product_variation->set('price', $price);
$product_variation->save();

To get the price number and currency code you can use appropriate getters methods:

$price_number = $price->getNumber();
$currency_code = $price->getCurrencyCode();

You can also create a Price object by using the static method:

$price = Price::fromArray(['number' => '9.99', 'currency_code' => 'EUR']);

At the moment you can't configure the order receipt email subject. The subject is hardcoded in the OrderReceiptMail service.

There is an issue to fix this, but until that happens the only way to alter the email subject is to use the hook_mail_alter().

/**
 * Implements hook_mail_alter().
 */
function MY_MODULE_mail_alter(&$message) {
  if ($message['key'] == 'order_receipt' && !empty($message['params']['order'])) {
    /** @var \Drupal\commerce_order\Entity\OrderInterface $order */
    $order = $message['params']['order'];

    $message['from'] = 'noreply@example.com';
    $message['subject'] = t('Receipt for Order ID: @number', [
      '@number' => $order->getOrderNumber(),
    ]);
  }
}