Since Record count in more than 500 K so DE is used. New and Update is already specified in question.

Explanation
By using Tracking Extract, File Transfer, and Import activities from Automation Studio, you can extract the data from the subscribers who have had emails fail due to held status within the last 60 days [1][2], and then transfer the data into a data extension that can be used as an Entry Source for Journey Builder. This will allow you to create an SMS message that can be sent to the subscribers to ask them to update their email address.
Additionally, Automation Studio provides the ability to schedule and automate the process so that the data is regularly updated and the SMS message is sent on a regular basis. For further information, please refer to the following Salesforce documentation:
* Automation Studio Overview: https://help.salesforce.com/articleView?id=mc_as_overview.htm&type=5 * Automation Studio Activities: https://help.salesforce.com/articleView?id=mc_as_activities.htm&type=5 * Data Extensions Overview: https://help.salesforce.com/articleView?id=mc_es_data_extensions.htm&type=5 * Journey Builder Overview: https://help.salesforce.com/articleView?id=mc_jb_overview.htm&type=5

Explanation
To recommend a scalable business unit hierarchy for Northern Trail Outfitters and its three subsidiaries, the consultant should suggest one parent BU and three child BUs with their own Sender Authentication Packages.
This will allow each subsidiary business to have their own branding, content, and subscribers, as well as access to aggregate reporting and shared subscribers from the parent BU. Each child BU should have its own Sender Authentication Package to authenticate its email sends with its own domain and branding. References:
https://help.salesforce.com/s/articleView?id=sf.mc_co_business_unit_setup.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.mc_es_sender_authentication_package.htm&type=5