Rehub theme is the hybrid theme. It means it’s a Classic theme with some functionality from FSE themes. One of these functionalities is the option to create custom FSE templates and template parts for products.

If you are interested in how to add custom code areas and template parts inside Product page, check our documentation about code areas in product

You can also create 100% custom product template in Gutenberg. For this, create a New reusable template and select it as a Product Single page in template section