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- Does this feature consume high amount of data?
- Is it possible to enable this for certain apps only?
- What happens I have multiple devices with same app but different settings?
Depends on how much data the app has created. Whatsapp for example can use up multiple GBs of data if you have too many media files.
I am not sure, depends on the apps I guess. Whatsapp for example allows you to backup it's data. If you are looking for a more general solution on Android, I'd personally prefer to use Titanium backup that allows backing up apps and their data as well. Note that this could still take up a lot of disk space and also requires root.
I didn't understand this question. Can you give an example?