where android backup its data?
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.