Database connection onto an module i'm new into the world of Drupal, i'm a student comming from the Symfony ^^

I have create a module an i want to add an database connection for my module. In looking for an answer i have see the method getConnection of the Database class. 

I have tried this onto my controller

$connection = \Drupal\Core\Database\Database::getConnection('moduleunicef','password');


for connection at my database called moduleunicef, i have see i need to modif my setting.php file. 

/* /sites/default/setting.php*/
$databases['default']['default'] = array(
 'driver' =&gt; 'mysql',
 'database' =&gt; 'moduleunicef',
 'username' =&gt; 'theuser',
 'password' =&gt; 'password',
 'host' =&gt; 'localhost',
 'prefix' =&gt; '',


);

In result i have this error message (For security i have change the password and user ^^ )


  Drupal\Core\Database\ConnectionNotDefinedException: The specified database connection is not defined: root in Drupal\Core\Database\Database::openConnection() (line 366 of core/lib/Drupal/Core/Database/Database.php).
  
  Drupal\Core\Database\Database::getConnection('moduleunicef', 'password')
  Drupal\unicef42\Controller\PbRouteController->index()
  call_user_func_array(Array, Array)
  Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber{closure}()
  Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object)
  Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
  Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber{closure}()
  call_user_func_array(Object, Array)
  Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
  Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
  Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
  Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
  Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
  Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
  Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
  Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
  Stack\StackedHttpKernel->handle(Object, 1, 1)
  Drupal\Core\DrupalKernel->handle(Object)


So i have tried an over method into my controller

$connection = \Drupal::database();


the test of connection it's Ok but it's the base who i have input onto my setting.php file? or the drupal DataBase created for the install of the website?

I need some namespace onto my controller?
I'm using this one to swith and send a query 

in the settings.php add

databases['customer_db']['default'] = [
  'driver' =&gt; 'pgsql',
  'database' =&gt; 'drupal',
  'username' =&gt; 'drupal',
  'password' =&gt; 'drupal',
  'host' =&gt; 'xx.xx.xx.xx',
  'port' =&gt; '5432',
  'prefix' =&gt; '',
  'namespace' =&gt; 'Drupal\\Core\\Database\\Driver\\pgsql',
];


Then use when it's necesary

$db = \Drupal\Core\Database\Database::getConnection('default', 'customer_db'); 
print_r($db-&gt;databaseType());
$query = $db-&gt;query("SELECT * FROM customer");
...