Read product attribute from phtml file – Magento 2.4

I have created a simple phtml file to add a 3D viewer in the product page using the following code. I have also created a custom attribute for the product, called model_file_name in which I type the name of the glb file I want to display.

If I replace FILENAMEHERE with an existing file name, the 3D model is shown in the page, the only problem I have is passing a file name specific for every product.

I have found some examples about how to read an attribute, but none of them refers to the code included in a .phtml file. The sample below is using the ProductRepository, but the code does not work, the error is: Notice: Undefined variable: productRepository

The file is stored in the folder ./app/design/frontend/[CUSTOM_THEME]/theme/Magento_Catalog/templates/view/customtext.phtml

How do I read a custom attribute from a .phtml file ?

<?php 
  /*
  * Some code here! to read the custom product attribute name model_file_name
  * The following DOES NOT works:
  * @var $productRepository MagentoCatalogModelProductRepository
  */
  ?>
  <?php
  $product = $productRepository->get("PRODUCTSKU");
  $attributes = $product->getAttributes();
  foreach($attributes as $a)
  {
      echo $a->getName()."n";
  }
?>
<?php #echo $modelFileName ?>

<!-- Import the component -->
<script type="module" src="https://unpkg.com/@google/model-viewer/dist/model-viewer.min.js"></script>

<style>
    .3d-model {
        height: 100%;
        width: 100%;
    }
</style>

<!-- Use it like any other HTML element -->
    <p>3D Model: </p>
    <model-viewer style="width: 100%; height: 600px; top: 0px; background-color: 0xeeefff;" 
    id="3d-model" class="3d-model" alt="3d model" 
    src="http://vrmur.local/media/models/FILENAMEHERE" ar environment-image="" 
    poster="" shadow-intensity="1" camera-controls>
    </model-viewer>