On May 23, 2018 at 08:35, highfigh said...
They also told me that apps use more power when they start up, so I should leave them open.
This is (and has been for several years) the correct way to use an iPhone/iPad. iOS does a better job of managing power and data usage w/ apps in the background than you can manually.
I'll occasionally manually close an app that has weirded out but otherwise leave them running. Not counting iOS updates I haven't rebooted my iphone X in several months. Its easily the most stable and reliable smartphone I've ever owned (after some 2018 iOS patches...)