iPhones/iPads can either back up to iCloud automatically, or you can do a backup via iTunes (running on a Windows PC or Mac) by plugging it in. You could then choose to store those backups on the external or wireless HD.
Personally, I’d just use iCloud for device backups. You get 5GB per Apple ID so assuming each person has their own account that’ll be more than enough for everything except photos. 79p a month gets you 50GB otherwise.
For photos – you’d have to plug in to the Mac and copy photos off to the hard drive too. If you’re averse to paying, then Google Photos lets you upload as many photos as you like, although their free offering will do some compression. Not bad as a free extra place to store it all, just in case. You can run the app on each device and it can upload automatically.
Apple’s solution for iOS device backup is iCloud, and you can have a couple of terabytes of storage on there if you’re willing to pay for it. Step outside of that and you’re into cobbling together other stuff.