It's none of the two. Turning of the screen of an Android device will not hibernate it (thinking of a phone: What about incoming calls if the device would be hibernated?). Android handles things a little different.
In the best case, with the screen switched off Android enters a very energy-saving mode called "deep sleep". It will cycle down the CPU to a minimum, may switch off WiFi (and other things) completely, and other things. Apps may enable those ressources when needed by aquiring corresponding "Wake-Locks" (see: wakelock), which they (hopefully!) don't do too often, and (again hopefully) release when no longer needed.
Not having "hibernated" the system completely allows the system to quickly react on events like e.g. incoming calls, alarms, or the user switching on the display again. It simply needs to speed-up the CPU, and enable those ressources it "put to sleep", if needed.
Also keep in mind that Android devices (usually) don't have mechanical components like "spinning hard-disks", so if storage is not accessed, it does not "consume power" :)