To ensure subscriber data is appropriately segregated between the EMEA and AMER teams, the Marketing Cloud admin should separate regions into different business units and apply Subscriber Filters. This setup allows for distinct data management and access controls per region, aligning with security and legal requirements. By configuring business units specific to each region, the admin can effectively control data visibility and operations within those regional contexts.
Reference: Salesforce Help - Business Unit Setup

To determine the total number of emails sent across all business units in the last calendar year, the Marketing Cloud admin should use Analytics Builder > Reports > Email Send Report. This section of Analytics Builder provides comprehensive reporting capabilities, including the ability to generate detailed reports on email sends across different business units over specified time periods.
Reference: Salesforce Help - Email Send Reports

To effectively set up a welcome journey leveraging customer data across multiple data extensions in Contact Builder:
Single Attribute Group: Create one cohesive Attribute Group that interlinks Customers to Contacts, Orders to Customers, and Products to Orders. This setup ensures a streamlined data flow and relational integrity, crucial for targeting and personalization within the journey.
Data Relationships: This configuration facilitates the use of data across different stages of customer interaction-from acquisition through orders to product preferences-enabling more personalized journey experiences.
Reference: Salesforce Marketing Cloud Contact Builder

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

To ensure that users have access to Query Studio in Marketing Cloud, the admin should license all appropriate users within the installed package. This involves configuring user permissions and ensuring that licenses are assigned to users who need access to Query Studio. This approach ensures that only authorized users can use the tool, maintaining security and compliance within the organization.
Reference: Salesforce Help - Installing and Managing Apps in Marketing Cloud