Twig

Example 1. Basic inline template

    $output[] = array(
      '#type' => 'inline_template',
      '#template' => '{{ yourvar }}',
      '#context' => array(
        'yourvar' => 'The Value',
      ),
    );


Example 1, with FieldPluginBase (\Drupal\views\Plugin\views\field\FieldPluginBase)

Step 1. Create your module and Block (/tutoriels/107/create-a-simple-block-programmatically)

Step 2. Add your template usin hook_theme()  (here module name = clock)

Enable debugging

You enable Twig Debugging in sites/default/services.yml.
(If services.yml does not yet exist; copy default.services.yml and rename it to services.yml.)

parameters:
 twig.config:
  debug: true

Printing variables

Print : {{ variable_to_print }}
Code : {% codes %}
Comment : {# Comments #}
 

Official guides:

https://www.drupal.org/theme-guide/8/twig
https://www.drupal.org/theme-guide/8