Most overage charges come from data you never chose to spend. Nine Android settings stop it, and the last one tells you if you're on the wrong plan.
Settings > Network & internet > SIMs > App data usage ranks every app by megabytes used this cycle. One or two apps usually explain most of the bill.
In the same SIMs menu, open Data warning & limit and turn on Set data limit. Your phone switches mobile data off at the number you pick, so overage never starts.
Settings > Network & internet > Data Saver lets most apps get background data only over Wi-Fi. Anything you need live goes on the Unrestricted data list.
Tap an app inside App data usage and turn Background data off. It still works normally when you open it, it just stops syncing on mobile behind your back.
Netflix says Maximum Data burns a gigabyte every 20 minutes. Save Data stretches that same gigabyte to roughly six hours. It sits in the app's playback settings.
In YouTube, tap your profile photo, then Settings > Data saving. Video quality preferences sets a separate default for mobile networks and for Wi-Fi.
Play Store > profile > Settings > Network preferences > Auto-update apps > Over Wi-Fi only. App updates are the quietest gigabytes on any phone bill.
In Google Photos, tap your profile photo > Photos settings > Backup > Mobile data usage. Video backup over cellular is the switch worth turning off first.
Settings > Network & internet > SIMs, then turn Roaming off. Leave it off abroad until you know what your carrier charges. That is where horror bills start.
After one full cycle, compare App data usage with what you pay for. Google notes carriers count differently, but a wide gap means you are buying too much.