Why does my iPhone churn through data?

For starters, I have two phones. I Think it's really important to have a back up, so I purchased an Amazon fire phone right after they flocke Think it's really important to have a backup, so I purchased an Amazon fire phone right after they flopped. I wanted a backup, and I thought it looked cool, I love Amazon, and most importantly it was really cheap. When I first bought it I was still using my iPhone, so I checked my data usage at the end of the month and I had used 9 gigs. It was outrageous but I share a family plan with five people, and very few of them use that much data so of the 15 we have we only ended up using 11. This meant my parents just got mad at me. I plugged my sim into my fire phone and the problem disappeared. I was using like 1 fifteenth the amount of data I had been even though I was doing the same stuff. Now I have to use my iPhone again and IIm again, churning through data. I don't think anything is running in the background and I don't think my phone is corrupt but I need this to stop and I need it to stop now.

If you have a newer iPhone you can go to settings then cellular and scroll down to see what's using the most data.