When apps are in the background they are not running, they are just paused and sitting waiting and it's actually better to leave them there if you plan on going back to them in a little while, because each time you open an app it has to load everything from the simple things that never change to the stuff that does update and change often. Closing the app shuts it down completely so next time when you go to open it the app has to load every little thing back up and that takes more battery and effort for your phone so jt actually makes more sense to leave it open if you plan on going back to that app soon
More apps running it will running the battery. you always have like email and messaging apps always checking for updates. once you connected to the internet your phone always working and having plenty apps running it carrying a weight on the memory and also battery.
