android: How to monitor and control data usage on your Android phone
Android smartphones that run on Android 8.0 or above come with the Data Saver mode. This mode starts working when you’re not on Wi-Fi and ensures that apps and services that are not being actively used won’t be able to stream data in the background. Turning on the data saver mode might slow things down a bit, however, it can come in handy if you are concerned about crossing your data limit.
Source: gadgetsnow.com