How do I down load photos to backup drive?

You can use Photos to sync your iCloud Photos. Open Photos. Go to Photos > Settings > iCloud. You'll likely want the original photos (not optimized).

You can further move the photos library to an external drive: Move your Photos library to save space on your Mac - Apple Support

Regardless, you'll want a separate external hard drive to make a Time Machine backup. Back up your Mac with Time Machine - Apple Support

What would you like to do with these pictures that you're downloading to an external drive? For instance, I occasionally make a backup copy of my Library by just dragging it from the Pictures folder to a folder on an external drive. As muguy implies, this is probably only useful if "Download Originals" is checked in Preferences>iCloud--otherwise I'd be backing up lower quality images. Depending on the size of the Library, it might take awhile.

Sometimes I put pictures on an external drive or a flash drive to give to friends. Then, in Photos, I use File>Export>Export xx photos. These picture will include my edits, and can include titles and keywords.

If you want to have a Finder level backup of your pictures, you can go to an Album, select all, and Export. But you need to decide if you want the Original, unedited, unkeyworded pictures, or if you want the edited versions. When exporting, you will have to create a folder for each album.

You can select an album and Export>Album, but you get thumbnails doing that, apparently. Or Export>Album Originals gets you full sized but un-edited originals.

If you want more control over metadata, dates, and so on, then the 3rd party app PowerPhotos gives more options.

There are certainly more reasons and methods to export. Let us know if what you want to do...