Explanation
To send an email with 10 personalized fields from multiple data extensions to an audience of over one million subscribers, Northern Trail Outfitters should create and send to a sendable data extension containing those needed fields using a query activity. A query activity can join and filter data from multiple data extensions and populate a sendable data extension with the required fields for personalization. This will improve the email send performance and avoid timeouts or errors. References:
https://help.salesforce.com/s/articleView?id=sf.mc_as_query_activity.htm&type=5https://help.salesforce.com/s/a

Explanation
To export a data file to its SFTP containing all subscribers who did not receive their intended email in the last
24 hours, Northern Trail Outfitters should use a Data Extract Activity in their automation. A Data Extract Activity is an activity that allows marketers to extract data from Marketing Cloud and place it in a file on an Enhanced FTP location or an external SFTPlocation. The Data Extract Activity can use different extract types, such as Tracking Extract or Data Extension Extract, to export different types of data.
References:https://help.salesforce.com/s/articleView?id=sf.mc_as_data_extract_activity.htm&type=5

To format the JSON payload from their commerce platform within the email, Northern Trail Outfitters should use the BuildRowSetFromJSON() AMPscript function. The BuildRowSetFromJSON() AMPscript function is a function that allows marketers to parse a JSON string and return a rowset object that can be used to display data within an email message. The BuildRowSetFromJSON() AMPscript function can handle nested JSON objects and arrays. References: https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-programmatic- content.meta/mc-programmatic-content/buildrowsetfromjson.htm