Record admin changes in log?
In Drupal 8 configuration is also stored in files, so you can commit it to git and have a full log. As far as I know there is no mechanism to record all actions of users in any version of Drupal.

If you insist, you can implement hook_form_submit() for every form and add additional save function to record who, when and what, but it will be hard to use log created this way.

Other approach is to use Features - let them develop their parts on their copies and just send you packages with working pieces.