I wondered about my android's data usage rapidly reached the limit every
month. I hadn't known that many apps working with data in the
background.

Yesterday, i found great app on play store to see data usage used by every app on my phone, and also the percentage of the usage itself. The app is My Data Manager, you can download it for free from play store.
This app also allows us to set notification if it reaches certain used data. Very cool!
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