Among the several types of triggers available for you to use when designing your own automation workflows, the "When someone unsubscribes from any list" trigger is the type you need to choose, when you want to set an automation in motion as soon as someone unsubscribes from any of your mailing lists.
A. Setting the parameters of the trigger
The only parameter that you need to define on this type of trigger is whether the automation will be triggered for every unsubscription, or whether it will be triggered on the first unsubscription only. 
You can choose either option from the available drop-down menu, and after you're done, clicking on the Save button will place your trigger inside your workflow.
B. Taking advantage of the trigger
It's important to understand that no actions can be performed on members who have unsubscribed from a mailing list. Why have such a trigger if you can't perform actions on unsubscribed members? The answer is simple: you can subscribe them to other lists, where you can take advantage of all the available actions! First make sure that the unsubscribe settings on your Moosend account are set to: "Unsubscribe users from this mailing list, but send to them from other mailing lists".
Then you will need to create two separate automations, each one with its own trigger and actions:
- Automation #1: When someone unsubscribes from any list, subscribe them to list B.
- Automation #2: When someone is subscribed to list B (because of Automation #1), send them an email campaign, then unsubscribe them from list B.
Using the above idea you can design your own automations to perform actions on whoever unsubscribes. It'll be easy to contact your unsubscribes to say you're sorry to see them go, or to perform any other type of action from the ones available. Just make sure that you remove them from your second list afterwards, since they did request to be unsubscribed!