Q: how do I download this app (photos for mac)

It's part of OS X. If you don't have a backup you can restore from, re-install OS X.

Are you sure it's not in the Applications folder?  Photos is part of the system as KiltedTim pointed out and the system won't let you delete it. You get this message when you try to drag it to the Trash bin:

If you were logged in as root then maybe you could have deleted it.