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.