Hi Bart,
Basically, T4 Builder is kind of component, its content (elements/blocks) will display in the 'component' - main content block of a page so you need to have a menu item type of T4 Page Builder to show pages created by T4 Builder.
For articles, when you edit existing articles with T4 Builder, the existing content will be converted to T4 Builder content, however, there is issue with this feature as it's showing only elements/blocks from T4 Builder which you add into article. Our developer is working on this issue.
When you edit articles with T4 Page builder, basically it's still a Joomla article and it's stored in - xxx_content table in your database. Kindly check.