product field select element

[AnthonyWang]

VM 2.018
when i use firebug, i found it use select and option element

components\com_virtuemart\views\productdetails\tmpl\default_addtocart.php line:35
<span class="product-field-display"><?php echo $field->display ?></span>

is there any one know where i can find the select and option code in which php file?

I would be grateful for any help