Where is the configuration file for my custom plugins?
For configuration to be found as a file (yml) you have to manually invoke/export it with any of the following methods


from the Drupal admin interface visiting the url admin/config/development/configuration/single/export. For this to work the core Configuration Manager module must be enabled.
drush with drush config-export and than finding the appropriate file manually 
drupal console drupal config:export:single
contrib modules such as config_single_export or config_partial_export


All those methods above have also an import mechanism which you can use to import or see more details here

Also note you will not need the REST UI module to be enabled in production