This article explains how to create a Workflow Trigger: Email Events, what it does, and when to use it.
Click Add New Workflow Trigger.
This opens a selection panel on the right side of the screen.

Scroll until you see the Events column.
Click Email Events.
This trigger fires based on what happens within a particular email.

Click Add filters.

Under the Standard Fields dropdown, select Event.

Select the event type you want to trigger on:
Bounced: the email bounced
Clicked: a recipient clicked in the email
Complained: a recipient marked the email as spam
Opened: a recipient opened the email
Unsubscribed: a recipient unsubscribed from the email

You will have to specify which email. This only works if the email is part of another Workflow.

After the filter is added, click the dropdown ˅.
