Installing Drupal with existing Pantheon site?
To install Drupal on an existing Pantheon site you don't need to install or copy anything to your local computer; you can just work on the Pantheon development environment in the cloud using the dashboard workflow. Pantheon hosts three distinct environments for each site: development, testing, and the live production site. You can install Drupal plus modules and themes on your development site. Once you're satisfied with the results, you can push your site code to the test environment for user testing and then to the live environment to serve the site from your domain, once you've launched with a paid site. The workflow tools let you push code from dev->test->live, and also clone your database and files between environments. Typically you'll install Drupal and configure the modules on the development environment before launching the site, then clone that database up to live to retain all the content changes. However, post-launch you would only want to copy the live database and files down to development and testing environments, as the live production environment is the canonical source of site content. You'll want to review Pantheon docs for how to use the workflow, both pre-launch and post-launch:

https://pantheon.io/docs/articles/getting-started/