He/she needs to be added to the sites "Team" as "Team Member". See details in https://pantheon.io/docs/change-management/

You need to be part of the "Team". Access the site name and go to "Team" and add yourself(assuming you have access). Check back the aliases and you should have it now.

Your Username > Dashboard > Drush Aliases

See https://pantheon.io/docs/terminus/

Add the email to the Project's Team. The login credentials is the access to git as well. NOTE: If using Google login, make sure to change password.

See https://pantheon.io/blog/five-steps-feeling-drupal-drush

Pantheon.io hosted sites don't use htaccess. But there's an equivalent implementation via php. See https://pantheon.io/docs/redirects/