To use less mobile data, install and use your apps while the phone is connected to Wi-Fi. Wi-Fi is the wireless internet of a home or a public place.
Connect to a Wi-Fi network: at home, at the library, at a community centre. You should see the Wi-Fi symbol, the three lines in a V shape, at the top of the screen.
Do your installations and updates at that moment.
Turn on data saving: Settings > Cellular > Cellular Data Options > Low Data Mode (iPhone), or Settings > Network > Data Saver (Android).
Off Wi-Fi, avoid videos: they are what uses the most data.
If you don’t see it: menu names change from one brand to another. Type « data » in the search box of your settings.
Limitation: some apps do not work without internet. Their listing or their settings tell you whether an « offline » mode exists.
To picture it: mobile data is like the water in a bottle: Wi-Fi fills it, the cellular network empties it.