How do I transfer my photos in icloud to external hard drive

Pretty much any drive. Generally try to find one at least 1.5-2x the amount of space you need to backup. Eg. If you have a 1 TB hard drive in your Mac, look for at least a 2TB backup disk. Hard drives are cheap.

Ideally, you sync with the Photos app on your Mac and then backup your Mac to Time Machine.

Open Photos then Photos > Settings > iCloud to enable sync