I found this tutorial to set it for a category:
https://www.sellacious.com/documentation/page-builder/setting-up-sp-sellacious-add-ons

But how can I set this product page of SP Page builder as default for all single products?

That menu item is meant for a different purpose (SP Page Builder to be precise).

But I haven't understood how to show the product page of SP Page Builder when a product is shown. What is the procedure to be able to?

Hi Kelvin,

It is reserved for the Sellacious-SP Page builder integration. We might change the description or title in later releases to make it more precise. Thanks for suggesting this.

Pagebuilder has it's menu, there is no point of having it in Sellacious again. It's misdirection.

Hi Kelvin,

That menu item is meant for a different purpose (SP Page Builder to be precise). You can link a single product in menu by finding the product link from browser(e.g: /shop/laptops/thinkpads?view=product&p=P8V0S513) and using external link in Joomla menu system to link it.