The data siloing on iOS is a UI decision, not a sandboxing decision. They decided most people simply don't understand filesystems (probably based on all the people they've seen who just save their files in the default directory, or the desktop, or the recycle bin(!) and then can't find them)
In the macOS sandbox, an app can access any old file, but only ones the user has implicitly told the app it can access - through the system "open file" dialog, double-clicking, drag & drop, etc.
In the macOS sandbox, an app can access any old file, but only ones the user has implicitly told the app it can access - through the system "open file" dialog, double-clicking, drag & drop, etc.