When you use the Copy-Chats PowerShell command, the migration tool creates a SharePoint site collection and libraries with custom permissions to store your users' chat history.
The site and libraries ensure that only participants in a chat can see that chat's history.
Because of the nature of a site collection's permissions, any global admin, SharePoint admin, and site collection admin will have access to the entire chat history.
Caution: Avoid adding admins or modifying permissions in the destination site collection to protect the privacy of your users.
For more information, see Copy Chats.