Data storage location? Difference between server and physical device Just for my curiosity and knowledge, all the DATA saved into services like iCloud email, Gmail email, DropBox, iCloud Drive, etc.  Do they actually take physical space storage inside each iOS or OS X device? Or they are just a mirror of their server? Example iCloud email service I do know that is a part of the 5GB but does it also take space inside the physical device? When there is no WIFI or Cellular connection at all, emails are still in the devices. 

Do you also recommend a nice reading article about this topic?

Thank you so much and have a wonderful day.
On OS X, Dropbox and iCloud keep local copies of all the files you have stored on them. The reasoning being that Macs usually don't want for space like iOS devices do. On iOS, Dropbox doesn't store the files locally until you try to do something with one, at which point it's downloaded. iCloud handling depends on the application. I know for some applications, the files are kept locally. Pages for example will download all available documents, seeing as how text files are typically not that large. I'm not certain about the rest.