Inside WordPress dashboard, download Advanced Custom Fields (ACF) plugin. Inside ACF plugin, add a custom Relationship field to all posts under the custom post type 'Testimonials'
Stay in WordPress dashboard. Add new testimonial posts, and select mentioned instructor(s) or mentioned service(s) for each testimonial in the ACF fields we just added in step 2.
Now, we need to display Product-specific testimonials on individual Product (or Service) page. This page is a php file coming from the WooCommerce plugin, so we will copy 'content-single-product.php' from WooCommerce plugin files and paste to our own theme folder (root folder > woocommerce > content-single-product.php). We can edit there.
In our 'content-single-product.php' (individual service page), display the Product-specific Testimonials like so: