Hi adsoma
It's a good question when you start with T4 Builder, these are different builders which are implemented with different models.
There are some common things like they all offer built-in blocks / elements when editing a page and if you want to have a page looks like demo site, just import page from Page Library (JA Builder) or Bundles (T4 Builder).
JA Builder comes with a built-in template so when you install JA Builder package, it will have a Joomla template ready to use with JA Builder. If you want to combine with other Joomla template, it will require settings for menu item type and template assignment.
With T4 Builder, its kind of a 'component' and separated from template (T4 Blank, in this case). With being said, you can use T4 Builder with any template and things you build for a page: blocks, elements, ... will show up in the main component block of a page.
T4 Blank template is simply a Joomla template on your site and it'll be used on your whole website if you set it as default template. It will have impact on the website bundles that you import from T4 Builder. Because the priority of loading style for a page will be: T4 Builder blocks >> Joomla template >> Framework (if any, in this case is T4 framework) >> Joomla default style.
I hope this makes senses and let me know if you need any further help.