How to nest fieldsets?

My goal is rather simple, I need to group several fieldsets in a fieldset like in the image below :

I tried with the following code :

    public function buildPaneForm(array $pane_form, FormStateInterface $form_state, array &$complete_form) {
        
        foreach ($this -> order -> getItems() as $key => $order_item) {
            
            dpm($order_item -> getPurchasedEntity() -> getProduct() -> bundle());
            $quantity = round($order_item -> getQuantity());
        
            $pane_form['pilote_'.$key] = array(
                    '#title' => $order_item -> getPurchasedEntity() -> getProduct() -> getTitle(),
                    '#type' => 'fieldset',
                    '#description' => "Saisissez les coordonnées des pilotes à assurer",
            );
            
            for($i = 0; $i < $quantity; $i ++) {
                
                $pane_form['pilote_'.$key]['ss_pilote_'.$i]  = array(
                        '#title' => 'Pilote '.($i+1),
                        '#type' => 'fieldset',
                );
                
                $pane_form['pilote'.$key]['ss_pilote_'.$i]['pilote_nom_'.$i] = array(
                        '#type' => 'textfield',
                        '#title' => 'Nom',
                        '#default_value' => '',
                        '#required' => TRUE,
                        '#size' => 20,
                        '#attributes' => array(),
                );
                    
                $pane_form['pilote'.$key]['ss_pilote_'.$i]['pilote_prenom_'.$i] = array(
                        '#type' => 'textfield',
                        '#title' => 'Prenom',
                        '#required' => TRUE,
                        '#default_value' => '',
                        '#size' => 20,
                        '#attributes' => array(),
                );
                    
                $pane_form['pilote'.$key]['ss_pilote_'.$i]['pilote_email_'.$i] = array(
                        '#type' => 'textfield',
                        '#title' => 'Email',
                        '#default_value' => '',
                        '#required' => TRUE,
                        '#size' => 20,
                        '#attributes' => array(),
                );
                
            }
            
        };
        
        return $pane_form;
    }

but i still get this :

can anyone help me get the desired result ?

Thanks in advance for your answers :-)