New user, confused

Asking about both the Mac OS and iOS app’s. Why is the OS one free and iOS paid? I didn’t find a reason and how they both integrate. I am primarily wanting to backup to my 2TB iCloud account what I have normally been using a RAID for. What then, is the iOS scenario for?

The mobile apps are only needed if you want to access your vault on a mobile device. Think about them as an addon.
Why are they not „free“? See here: Why do I have to pay for Donation Key and Android App?

If you do not want to access the files in your vault (in your case your Backup) with a mobile device, then you do not need the mobile apps.