Explanation
To clear inactive data stored in data extensions after 12 months, NTO should apply a row-based retention to each data extension as it is created, set to 12 months. This will delete any records that have not been modified in the last 12 months. Row-based retention can be applied to individual data extensions or data extension templates

Explanation
Configure the business unit data retention setting to 12 months is the policy that the Marketing Cloud admin could configure in Setup. Business unit data retention setting is a setting that allows marketers to specify how long records in data extensions should be retained at the business unit level. By configuring the business unit data retention setting to 12 months, any inactive data that is older than 12 months will be automatically deleted from the data extensions in that business unit2

The behavior where customers unsubscribe via the Marketing Cloud default Profile Center and do not receive emails from any other business unit is explained by:
* D. Business unit unsubscribe setting causes an Enterprise level unsubscribe. This setting means that when a subscriber opts out using the default Profile Center, they are unsubscribed at the enterprise level, impacting all business units within the organization.