Drupal 8, how build a hierarchical form?

In Drupal 8, I don't understand how build a "hierarchical" form.

I have this sample form

...
public function buildForm(array $form, FormStateInterface $form_state) {
    $form['description'] = array(
        '#type' => 'fieldset',
        '#title' => t('Main description'),
    );
    $form['description']['subfirst'] = array(
        '#type' => 'textfield',
        '#title' => t('subfirst'),
    );
    $form['description']['subsecond'] = array(
        '#type' => 'textfield',
        '#title' => t('subsecond'),
    );

    $form['content'] = array(
        '#type' => 'fieldset',
        '#title' => t('Main description'),
    );
    $form['content']['subfirst'] = array(
        '#type' => 'textfield',
        '#title' => t('subfirst'),
    );
    $form['content']['subsecond'] = array(
        '#type' => 'textfield',
        '#title' => t('subsecond'),
    );

    $form['submit'] = array(
        '#type' => 'submit',
        '#value' => 'Submit',
    );
    return $form;
}

public function submitForm(array &$form, FormStateInterface $form_state) {
    dpm($form_state->getValues(),"getValues");
}
...

When I submit my form, my form_state->getValues() return :

The form_state->getValues() contains only the ['content']['subfirst'] and ['content']['subsecond'] values... That means I must use uniques labels with the form api ? I find it weird...

Then, I change my form :

$form['content']['subfirst'] become $form['content']['totosubfirst']

$form['content']['subsecond'] become $form['content']['todosubsecond']

The new code :

public function buildForm(array $form, FormStateInterface $form_state) {
    $form['description'] = array(
        '#type' => 'fieldset',
        '#title' => t('Main description'),
    );
    $form['description']['subfirst'] = array(
        '#type' => 'textfield',
        '#title' => t('subfirst'),
    );
    $form['description']['subsecond'] = array(
        '#type' => 'textfield',
        '#title' => t('subsecond'),
    );

    $form['content'] = array(
        '#type' => 'fieldset',
        '#title' => t('Main description'),
    );
    $form['content']['totosubfirst'] = array(
        '#type' => 'textfield',
        '#title' => t('subfirst'),
    );
    $form['content']['totosubsecond'] = array(
        '#type' => 'textfield',
        '#title' => t('subsecond'),
    );

    $form['submit'] = array(
        '#type' => 'submit',
        '#value' => 'Submit',
    );
    return $form;
}

And when I submit my form, my form_state->getValues() return :

I get the four values. But, they are in the same hierarchical level. How I use the form api for have a form_state like this :

'description' => 'subfirst' => string(3) "AAA"

'description' => 'subsecond' => string(3) "BBB"

'content' => 'totosubfirst' => string(3) "CCC"

'content' => 'totosubsecond' => string(3) "DDD"

?

I want get a hierarchical form_state because after I want create a custom function like :

foreach element in description
  do that
foreach element in content
  do that
...