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.