OK. I really only know about half of what I'm doing here but it may get you started.
components/com_content/views/category/tmpl/projectdefault_children.php - duplicate of default_children
components/com_content/views/category/tmpl/projectdefault.php - duplicate of default.php
components/com_content/views/category/tmpl/projectdefaul.xml - duplicate of default.xml
layouts/joomla/content/category_projectdefault.php - duplicate of category_default.php
You also need to edit these files that you duplicated and change "DEFAULT" to "PROJECTDEFAULT"
For more details on creating an override, see: https://docs.joomla.org/Layout_Overrides_in_Joomla