I want to download all my I phone Photos from Cloud to an external hard drive

Manage iCloud storage from Mac Manage your iCloud storage - Apple Support then transfer photos from iPhone to Mac / PC Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support

Photos can’t be transferred directly from iCloud to an external hard drive for that you need to set up iCloud photo library see the article Set up and use iCloud Photos - Apple 

And its preferences Change preferences in Photos on Mac - Apple  open finder > Go > home > your user name > pictures either drag and drop Photos Library.photoslibrary to an external hard drive , the second method is right click on Photos Library.photoslibrary and click on show package contents in the upcoming column , you can also drag and drop complete masters folder to an external hard drive that is connected .

Also as per the article Back up the library in Photos on Mac - Apple 

Manually copy your library to an external storage device: Drag the Photos library (by default in the Pictures folder on your Mac) to your storage device to create a copy.See this article also

Photos library - Move your Library to an external storage device

Enable iCloud Photos on your laptop to sync all of your photos.

PC: Set up and use iCloud Photos on your Windows PC - Apple Support

Mac: Set up and use iCloud Photos - Apple Support

Once synced, copy the photos that you wish onto an external drive.