how do I move my photos and videos on my iCloud to my external hard drive?

To transfer photos from iPhone to Mac see the article Transfer photos and videos from your iPhone, iPad, or iPod touch - 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 these articles also .

Photos library - Move your Library to an external storage device

Use external storage devices with Mac - Apple Support