How to display Customer Group Name

You can utilise the Customer Group Specific settings to achieve this directly from the Magento admin:

Alternatively in your theme's copy of the template file you want to adjust add the below code

<br/>Customer Group Id: <?= $order->getCustomerGroupId() ?>
<?php if ($order->getCustomerIsGuest()): ?>
    <br/>GUEST ORDER
<?php else: ?>
    <!-- Alternatively move this into a viewModel -->
    <br/>Customer Id: <?= $order->getCustomerId()?>
    <?php $groupRepo = \Magento\Framework\App\ObjectManager::getInstance()->get('\Magento\Customer\Api\GroupRepositoryInterface'); ?>
    <?php $group = $groupRepo->getById($order->getCustomerGroupId()) ?>
    <br/>Group Name: <?= $group->getCode()?>
<?php endif; ?>