This gist is used in the tutorial https://www.vijayan.in/how-to-add-quantity-input-in-shop-page

quantity_update.php

<?php
add_action('wp_footer', 'update_quantity_attribute');
function update_quantity_attribute()
{
?>
    <script type='text/javascript'>
        let simpleProductInput = document.querySelectorAll('.product-type-simple .quantity > input');

        simpleProductInput.forEach(dom => {
            dom.addEventListener('input', e => {
                e.target.parentElement.nextElementSibling.setAttribute('data-quantity', e.target.value);
            });
        });
    </script>
<?php
}