Email coming into HighLevel from an external software, how does the CRM know which Contact record to update?

Usually the Contact in a Workflow is determined by the sending email address. But in this case it would be the system email address (AP@no-reply<dot>system<dot>biz, or, sys@no-reply<dot>system<dot>biz).

What I want the Workflow to do is read the email (parse the data to find the relevant Contact in the database then perform all subsequent actions on that Contact instead of the initial (system) Contact. Alternatively, initiate another Workflow with the input info from the system email but acting on the human instead of system Contact record.

Use Case 1: email coming from the external system has Contact ID, Opportunity Name, and Contact email along with other relevant data and a link to a password protected data repository. Needs to update the Contact record and Opportunity card and send an email to the Contact with the link.

Use Case 2: email coming from the external system is to the Contact email and cc to the HighLevel account. Needs to update the Contact record with a copy of the attached pdf in Documents.