How do I migrate files to Google Shared Drive?

All content inside your Shared drive is automatically shared with the entire Shared. 

You cannot move a folder from My Drive to a Shared Drive.  You must move the files, though you can move multiple files at one time.

A file’s permissions and sharing link will not change when it’s moved into a Shared Drive. People who aren’t Shared Drive members will still be able to access that file with their previously granted permissions. But, everyone in the "Shared" will also have access.
  1. If you are given permission you can move any file you own into a Shared Drive, whether it’s from another Drive location, or from your computer or mobile device.
  2. To move files between Shared Drives or from My Drive into a Shared Drive, drag the files into the destination Shared Drive folder.

         a. If someone else owns the file, you need to ask them to move it.