How to retrieve #tree values inside a form?

I am new at Drupal 8 and I am creating a module that has a #tree form. I chose #tree form to create multiple forms which has the same textfields and button.

I came across one document in drupal about #tree.
https://www.drupal.org/docs/7/api/form-api/tree-and-parents
I have made a researched about it and can't find any documentation, post or blog on how to get a specific value from #tree.

I have implemented the #tree in my form. The only problem is that I can't retrieve a specific value from the form.

for($counter = 0; $counter < $rowCount; $counter++){
   $form['firstname']['#tree'] = TRUE;
   $form['firstname'] => [
        '#type' => 'textfield',
        '#title' => 'First name',
   ];
   $form['secondname'] => [
        '#type' => 'textfield',
        '#title' => 'Second name',
   ];
   $form['save'] => [
        '#type' => 'submit',
        '#value' => $this->t('Save'),
        '#submit' => ['::submitForm'],
   ];
}

The rowCount pertains to the row in my database. The process is to acquire firstname and lastname from the user and those variables will be saved inside the database.

How can I get the 2nd firstname inside the #tree form?

#tree Creates a hierarchy of values in the submit handler of a form.

Compare:

public function buildForm(array $form, FormStateInterface $form_state) {
  $form['container'] = [
    '#type' => 'container',
  ];
  $form['container']['some_value'] = [
    '#type' = >'textfield',
  ];

  // Submit buttons etc. not shown

  return $form;
}

This will create a form element. After clicking submit, the key of the form element is used to return the value. in this case, the key is some_value, meaning validation and submit handlers can retrieve the submitted value with form_state->getValue('some_value').

Now, add #tree to the container:

public function buildForm(array $form, FormStateInterface $form_state) {
  $form['container'] = [
    '#type' => 'container',
    '#tree' => TRUE,
  ];
  $form['container']['some_value'] = [
    '#type' = >'textfield',
  ];

  // Submit buttons etc. not shown

  return $form;
}

With this change, $form_state->getValue('some_value') will return nothing, as the value is now part of the container element (the #tree element) and needs to be retrieved relative to that:

// Returns an array with a single key, 'some_value', that contains the
// submitted value.
$form_state->getValue('container')

// Returns the submitted value:
$form_state->getValue(['container', 'some_value'])

#tree denotes a hierarchy in the form, you still need valid and logical PHP to make it work, e.g.

// In the build...
$form['parent'] = [
  '#type' => 'container',
  '#tree' => TRUE,
];

for($counter = 0; $counter < $rowCount; $counter++){
  $form['parent'][$counter] = [
    'firstname' => ['#type' => 'input', ...],
    'lastname' => ['#type' => 'input', ...],
  ];
}

// In the submit...
// '0' is the key you set with $counter previously.
$first_firstname = $form_state->getValue('parent')[0]['firstname'];
$first_lastname = $form_state->getValue('parent')[0]['lastname'];