Macs come with an app called Image Capture that can also be used to move photos from an iPhone to any folder on your computer including one on an external drive. This would be exclusive of using the Mac Photos app, however. You plug in your phone while Image Capture is running and then you can drag photos out to a folder or use the app to point to a folder and transfer the photos to it. There is also an option to delete the files you copy from your phone, if you so choose.
If you do decide to use Mac Photos, you can just plug in your phone while Photos is running and it will show up in the sidebar to allow you to import the photos into the Photo Library. From there you can export them to a folder on an external drive if you need to.
You might also consider using iCloud Photos which automatically syncs photos between your devices so you don't even have to connect your phone to your Mac.