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.
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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.