I don't have an iPhone, but the tricks they use will be similar to Android.
When I say tricks, I mean the way they use apps to track and monitor you. The way they use apps to record every single phone call and every single text you send. These apps, some of them hidden, use up a lot of memory and data.
I have everything on my android turned off as much as possible. There are tutorials online. For a new phone is takes me a couple of hours to set up. My phone still functions ok with this stuff switched off, but I don't use it for games or google accounts.
For most people, a phone will use them far more thean they use it.
