SPPB works with separate content to Joomla articles. So you can't open a Joomla article, as such with SPPB.
I'm not exactly sure what you're asking, but once you create a page in SPPB, you link it like any other menu item through Joomla's menu manager.
You'd start with a new page, and add elements (addons) of SPPB to the page, such as a Contact Us web-form.
Also if it's easier for accessing the editor, create a menu item in the front-end of your site called "Editor" (or something like that). Given the menu item "Super user" permissions (or other, as relevent to your situation).
Log in to the front end of your site. Click that menu item. The SPPB interface will load, listing the pages, and give you the ability to create additional pages.
Is that what you were asking about or not being able to find?
Even so, when I lauch SPPB from the back-end it will load the same page list once I get past the "Get started" screen.
Not sure if I have addressed your question(s) but I would also recoommend you read the documentation.
https://www.joomshaper.com/documentation