The method ReviewReminder::sendReminder() is the code which actually constructs and sends the message. The variable $email is a SubmissionMailTemplate and its methods are used to set the properties of the mail, like the Subject, From, etc.
You’ll find methods ccAssignedSubEditors() and bccAssignedSubEditors() in SubmissionMailTemplate:
I’m not immediately familiar with what it means to be a “Sub Editor” in OJS 3. If this role is what you want to target, these functions are for you.
These methods require a $submissionId and a $stageId as parameters.
An example of getting the appropriate submission id is in the same calling method:
But we’ll need to look elsewhere for the value of the $stageId. There are defined in PKPApplication:
The submission itself can return its current stage via Submission::getStageId():