“Force Closing Apps After You Use Them Saves Battery Life”
There are many types of apps, and they don’t all work the same nor use the same amount of resources. Some apps with background activities (processes that run even when you aren’t actively using the app) do slowly drain battery life, but you can turn off Background Activity functions or force close the app (on iPhone, that’s the swipe-up option in the App Switcher) to stop that.
Other apps don’t take much battery life at all, and going through to shut them all down probably won’t make much difference to your overall battery life. Even worse, if you frequently use an app, fully shutting it down and starting it up again every time will use up more battery life than if you just left it stagnant in the background.