hello.module

<?php
//Console debug output
//file_put_contents("php://stdout", sprintf("test")); 

function hello_theme() {
  return array(
    'hello_text' => array(
      'template' => 'hello-text',
      'variables' => array('text' => NULL)
    ),
  );
}

use Drupal\Core\Block\BlockPluginInterface;

function hello_block_view_hello_block_alter(array &$build, BlockPluginInterface $block) {
  $build = array('#theme' => 'hello_text', '#text' => 'you have been hooked');
}

use Drupal\Core\Form\FormStateInterface;

function hello_form_hello_form_alter(array &$form, FormStateInterface $form_state, $form_id) {
  $form['hello_config']['#description'] = t('This hooked text will appear in the form.');
}

//hook_menu_alter does not exist any more in D8
function hello_menu_links_discovered_alter(&$links) {
  // do not use route name but route ID
  $links['hello.main']['title'] = 'Hello hooked';
}