SharePoint migration tool and Active Directory Groups

Hello,

If you decide to preserve user permissions, the permissions on the target library will be replaced by that in source root folder. Existing files -in destination- with Unique permissions won't be changed. Migrated files without any permission in the source will have inherited permissions and inherited role assignments from the target library. Migrated files with any permissions in the source will carry over these permissions as unique.

There could be multiple behaviors depending on the parameters used for the migration, and the destination (root folder, subfolder, etc.). I'll ask you to take a look at this article for additional information: https://docs.microsoft.com/en-us/sharepointmigration/understanding-permissions-when-migrating.

Have fun!