If you want to add it to product card in shop page you need to override products_item template in website_sale module and If you want to add it to the product page you need to override product template in website_sale module.

Example to add internal reference in product page before description sale field:

 
<template inherit_id='website_sale.product' id="product_internal reference"  name="Product Internal Reference">

          <xpath expr="//p[@t-field='product.description_sale']" position="before">

            <p t-field="product.default_code" class="text-muted my-2" placeholder="A short description that will also appear on documents." />

        </xpath>

</template>

Does this work for odoo 16

I added the code below to templates.xml in the website_sale module

<template inherit_id='website_sale.products' id="product_internal_reference" name="Product Internal Reference">
<xpath expr="//div[='product.name']" position="inside">
<p t-field="product.default_code" class="text-muted my-2" placeholder="A short description that will also appear on documents." />
<t>template</t>
</xpath>
</template>

Why do you need this? may I have an use case?