Code

1. Define route in module.routing.yml

Exemple :

module.view.settings:

  path: '/url/to/the/page'

  defaults:

    _controller: '\Drupal\module\Controller\Demo::view'

    _title: ''

To set configuration at the module installation



Exemple:

$site_name = \Drupal::config('system.site')->get('name');

// Set the site name.

\Drupal::configFactory()->getEditable('system.site')->set('name', 'My site')->save();

You can do that using Symfony JsonResponse.

Exemple 1.

return new \Symfony\Component\HttpFoundation\JsonResponse($array);

Exemple:

Here an exemple : Module name : mymodule

Here an example to get the current page path programmatically in Drupal 8

    $current_url = Url::fromRoute('<current>');

    $path = $current_url->toString();

Exemple : Field 'username'

Store data on session.

Example:

$session = new \Symfony\Component\HttpFoundation\Session\Session();

$session->set('var_name', $session->get('var_name',0) + 1);

drupal_set_message($session->get('var_name'));

There are several ways to do that.

Using entityQuery, you can get the taxonomy terms  of a particular vocabulary.