Thanks to you all, I've just started to manage battery life with those tips.
Mostly, you've just started to think that you're managing battery life with those tips.
As the others said, no app is using your battery, except when it's on-screen, with just two exceptions: apps that use "special" functions (like streaming audio or GPS tracking) and apps that have background tasks. As a5cent wrote, those "background tasks" aren't like apps in the background on Android or desktop operating systems. Instead, they are specialized "parts" of apps that operate under very
strict rules about memory usage, CPU usage, and bandwidth usage. At worst, each is running for no more than 50 seconds every hour and are typically doing it with the screen off since most of the time the phone is in your pocket.
The best way to save battery is to turn off your screen. Most of us have the screen off when we aren't doing anything. Gaming, which constitutes a lot of screen-on usage, uses the CPU heavily. Those two things, screen backlight and CPU, are the biggest consumers of power on your phone. Managing the time that you have those things active will make a far bigger difference in overall battery life than tweaking background task permissions and closing suspended apps.
Sadly, in the end, most battery saving tips come down to "don't use the device as intended." Oh, well, life is full of compromises.