Can I add elements to a field widget with AJAX?

I want to add fields depending on the first option selected. I also made a form with a similar behaviour but it wasn't a field widget.

I used this module as an example to make the form

I'm not sure if the way I implement this AJAX behavior should be different in a widget.

Can the behavior of this form be implemented in a field widget? I'm trying to look for examples of specifically field widgets implementing this but I still haven't found anything that could help `

 public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $value = isset($items[$delta]->value) ? $items[$delta]->value : '';
    $element = [];

    // Gather the number of names in the form already.
    $num_names = $form_state->get('num_names');
    // We have to ensure that there is at least one name field.
    if ($num_names === NULL) {
        $name_field = $form_state->set('num_names', 1);
        $num_names = 1;
    } 

    $element['breakpoints'] = [
      '#type' => 'select',
      '#title' => $this->t('Breakpoint group'),
      '#options' => $this->getBreakpointsGroups(),
      '#default_value' => $value,
      '#empty_option' => $this->t('-Select a breakpoint group-'),
    ];   



    $element['images_fieldset'] = [
        '#type' => 'fieldset',
        '#title' => $this->t('Load images for each breakpoint'),
        '#prefix' => '<div id="breakpoint-wrapper">',
        '#suffix' => '</div>',
    ];

    for ($i = 0; $i < $num_names; $i++) {
        $element['images_fieldset']['image'][$i] = [
            '#type' => 'image',
            '#title' => $this->t('Image'),
            '#name' => "Name"
        ];
    }

    $element['images_fieldset']['submit'] = [
        '#type' => 'submit',
        '#title' => 'Submit',
        '#submit' => ['::submitForm'],
        '#value' => $this->t("submit"),
        '#ajax' => [
            'callback' => '::loadImageFields',
            'wrapper' => 'breakpoint-wrapper',
        ],
    ];


    return ['value' => $element];
}

/**
 * Returns a list of breakpoints.
 *
 * @return array
 *   An associative array of breakpoints, suitable to use as form
 *   options.
 */
protected function getBreakpointsGroups() {
    $breakpoints = \Drupal::service('breakpoint.manager')->getGroups();
    $breakpoint_names = array_keys($breakpoints);
    $breakpoint_group = array();
    foreach ($breakpoint_names as $name) {
        $breakpoint_group += [$name => $name];
    }
    return $breakpoint_group;
}

protected function submitForm($form, &$form_state) {
    $state = $form_state->getTriggeringElement();
    $breakpoint= $state['#value'];
    $array = array();

    $breakpoint = \Drupal::service('breakpoint.manager')->getBreakpointsByGroup($breakpoint);
    $breakpoints_name = array_keys($breakpoint);

    foreach ($breakpoints_name as $breakpoint_definition) {
        $mediaQuery = $breakpoint[$breakpoint_definition]->getMediaQuery();
        array_push($array,$mediaQuery);
    }      

    $name_field = $form_state->get('num_names');
    $form_state->set('num_names',count($array));

    $form_state->setRebuild();

}

public function loadImageFields(array $form, FormStateInterface $form_state) {
    return $form['images_fieldset'];
 }

}

The point is, you can't modify Drupal's internal form structure in an Ajax callback. So you'll need a submit handler, either for the field or for the complete widget, do the form modifications there and add a $form_state->setRebuild().

Ajax callbacks basically modify the HTML in the browser, that's all they do. They are called after the validation and submission has run.