After successful login the control panel from wp-content/themes/startuply/admin/index.php will be included on this page and this is the main page for your startup, the very heart of it.
Anything can be done with the above mentioned file but it is supposed that users will build a completely custom website in /admin directory in HTML/PHP/JS/Any framework/etc that is not using any WordPress functionality of the
original theme apart from user login.