Disable front-end access I want to disable the front-end access of my headless Drupal site, that is just a REST API for a Vue site.

Except for the login page, there is no need to access any node, taxonomy or view directly as everything is delivered via REST.

How can I achieve this?
Based on your responses in the comments, it sounds like you want anonymous users to be sent to the login page on any access and for authenticated users to be able to access the site as normal.  You can achieve this with a custom module that checks for logged-in status and redirect to the login page, or via the Require Login Module which does essentially the same thing.