Hi,
an unsubscribed lead will automatically not receive anymore emails.
When you for example sendout emails using a smart campaign and go to the "Schedule" tab you will see the number of leads who are affected by this campaigns and directly under this information all leads who are "blocked from mailing" which for example also contains unsubscribed leads.
They are also automatically added to the list "Unsubscribed Leads" in the Lead Database.
When you want to add them to one more list (I don't see a reason for that) you can build a smart campaign or smart list to do that. (Filtering them in the Smart List and if you want to add them to a list just use the Flow option "Add to List".
Hope that helps.