Not everyone has the luxury of an unlimited data plan. Google published a new app back in June, called 'Triangle,' designed to help users micromanage their cellular data usage. Now the app has left beta and become available worldwide, under the new name 'Datally.'
As far as functionality goes, it hasn't changed much since the initial beta. The app uses a VPN to determine the data usage of every app on your phone, and allow you to block background data usage on a per-app basis (Android 7.0+ can do this through the Settings, but the option is a bit hidden). It also provides metrics about your overall data usage, and an option to locate nearby user-rated Wi-Fi networks.
While the app is still clearly targeting developing countries, it's now available worldwide. You can get it from the Play Store at the link below, or from APKMirror here.