A video isn't really the best way to learn how to create a symlink. You can read these instructions which involve copying/pasting and editing to suit your setup - https://bluebellflora.com/saving-your-sims-4-folder-to-an-external-drive/
The assigned drives and problems unplugging and plugging in drives that @roberta591 refers to is only relevant to Windows, not macOS. There are also no invisible files or folders. After successfully creating a symlink to point to the external drive you will have a folder called Electronic Arts in the Documents folder on your Mac but it will be an alias with a little arrow in the corner showing this.
