To create custom trainable classifiers in Microsoft Purview, the user must have rights in the compliance portal. The Compliance Administrator role provides the necessary permissions to create and manage trainable classifiers. Security roles focus on threat management, and Global Administrator is excessive (not least privilege).
Reference: Trainable classifiers in Microsoft Purview

Explanation:
Box 1: The Insider Risk Management Investigators role allows users to view recommendations related to insider risk cases and Microsoft Purview DSPM for AI insights. This role is appropriate because it grants access to review AI-related risk recommendations without unnecessary administrative privileges.
Box 2: The Insider Risk Management Analysts role allows users to analyze user risk levels and events using Activity Explorer. This follows the principle of least privilege, ensuring that User1 can only view risk levels and investigate but does not gain full administrative control over insider risk policies.

To prevent the contents of files stored in Channel1 from being included in Microsoft 365 Copilot responses and ensure unauthorized users cannot access them, you should use Microsoft Purview Sensitivity Labels.
Sensitivity labels allow you to classify, protect, and restrict access to sensitive files. You can configure label- based encryption and access control policies to ensure that only authorized users can access or interact with the files in Channel1. Microsoft 365 Copilot respects sensitivity labels, meaning if a file is labeled with restricted permissions, Copilot will not use it in generated responses for unauthorized users.