According to the Google Workspace Updates blog1, the steps to prevent users from copying data from work to personal apps on iOS devices are:
Turn on "Advanced Mobile Management." This is a prerequisite for using the data protection settings1.
Navigate to Devices > Mobile and Endpoint > iOS Settings > Data Sharing > Data Protection1.
Clear the "allow items created with managed apps to open in unmanaged apps" checkbox. This will prevent users from opening files created with Google apps in their work account in any other app1.
Clear the "allow users to copy data to personal apps" checkbox. This will prevent users from copying content from a Google app in their work account to a Google app in their personal account or a third-party app1.