I understand you have an ISP with a limited data plan, and that you are interested in determining which apps or processes are using such an excessive amount.
Determining the cause is not a straightforward task without the help of third party utilities that I don't necessarily recommend. If you don't know where to start, EtreCheck may help. A description and download link are here: How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report - Apple Community.
Of course downloading it will just to add to your undesired data consumption, but only once. It's approximately 13.5 MB in size. It does exactly nothing when it is not running, and can just be ignored or dragged to the Trash when you're finished with it. Post its report in a reply to this Discussion, in accordance with the above instructions.