Where is the index file?
Drupal is a dynamic application, having an index.html file wouldn't make much sense.

There is an index.php file at the root of the install (the one you've already found). All requests are sent to that script, and the Drupal bootstrap takes over and routes them to the PHP function responsible for serving the appropriate content.

If you want to change the content on the front page, the simplest way would be to create a new node (of whatever type you feel is appropriate), visit /admin/config/system/site-information, and set the "Default front page" to the page of that new node.