Maybe iCloud is intended to mystify, to make users think that it works by Apple magic. It certainly behaves at times as if it has a mind of its own, one which doesn’t communicate very clearly what it’s up to.
I have two Macs which share my iCloud Drive: my main working iMac running Sierra, and a MacBook Air running High Sierra. They behave quite differently with respect to the contents of iCloud Drive, in that the iMac seems to keep a complete local copy of that, so copying a file from it is almost instantaneous. But the MacBook Air leaves larger files in the cloud, only downloading them on demand. This is not a behaviour over which I seem to have any control.
Despite that, iCloud likes to give the impression that all files are actually stored locally.
A couple of days ago, I was taking screenshots on the MacBook Air, using apps and documents which I had copied across to iCloud Drive from my iMac. Some of the documents were reasonably hefty, over 40 MB, and the MacBook Air left them in the cloud until I wanted to copy them across to its internal SSD.
When I copied those files, instead of seeing a progress dialog reporting that they were being downloaded from iCloud, the dialog claimed that it was “preparing to move” the document in question. These preparations took some considerable time, during which the Finder’s status bar revealed what was actually taking the time: it was “downloading 1 item”, and reported its progress in doing so. For much of that time, the progress bar in the dialog showed that its ‘preparations’ were actually complete.
iCloud’s logic is that the contents of my iCloud Drive are actually on both Macs, although on my MacBook Air the larger files are really only smaller placeholders. When I want to access one of the files for which only a placeholder is kept locally, iCloud replaces that placeholder with the full file, downloaded from iCloud. That process is termed preparing, even though it is actually downloading.
Without the Finder’s disarming honesty, few users would think that this delay was caused by any ‘preparations’ other than downloading. It’s hard to see why Apple has tried to make this obscure, other than to build an air of general mystery about what is taking place. Apple should replace that progress dialog with the same message given in the Finder status bar, and make the progress bar relate to the progress of downloading.