where android backup its data? For some reason, I had to reset my nexus5. Thankfully, I had "Backup my data" on. So, most of my settings are back. But I need to access the backup to change the wallpaper position (it has reset my last wallpaper, but I need to slide it). I have checked google drive and g+ auto-backup which backs up the pictures, but the wallpaper is not there.
Any idea where I can found the data backed up?
The data backed up using "Back up my data to Google's servers" aren't user-accessible. Each app has access to its own backup data. That's the only access available without hacking into the backup service.

This is a deliberate design decision. Android protects app data from being seen or modified by users or other apps, making it easier for apps to rely on the integrity of their data. (That is, unless you have root, in which case you're on your own.) Allowing users to intercept and maybe modify the backup data would defeat that protection, making it harder to write robust apps and harder for apps to restrict how you use their data (e.g. for movie rental apps).

You can learn more by reading my detailed answer about Google backup and how you can interact with it.