The WooCommerce Builder is a feature meant to boost your e-commerce experience by creating unique product pages for your individual products.
To use it, navigate from your admin dashboard to Product > All Products and pick the desired one.
On the right side you will see a section that contains the Enable WooCommerce Builder button.
You will be redirected to the WooCommerce Builder, similar to the Blog Post Editor.
From this point, you can start adding blocks and customizing the sale page of the product.