Wait, iCloud works on non-macOS/iOS? I didn't know that, that's good to know (Apple would be the first company I consider using despite not having client-side encryption: they seem likely to actively thwart dragnet surveillance).

Still, do they do anything that Dropbox/GDrive/OneDrive/etc. don't, like, does it actually do backups, or is it just another folder on your computer that my mom would have to drag new files into (similar to the external hard drive problem I described in the question)? The only place "backup" is mentioned on apple.com/icloud is with a screenshot of an iOS setting to back your device up. Which would be perfect, iff she used macOS.