I see too many people force closing apps after every use, their argument is that it will save battery, increase the devices speed, etc. Sometimes this results in the opposite, lose of battery, particularly with apps that are frequently visited, every time the app is forced to close it is removed from the RAM, thus every time it is opened the phone has to work harder to get it back in there. This is a popular misconception, especially when the term ‘multitasking’ is used to refer to backgrounded apps. If you are still concerned about this, you are better off managing your ‘Background App Refresh’ settings (iOS: Settings-> General-> Background App Refresh, Android: Settings->Data Usage) from there you can turn off which apps are allowed to refresh in the background or turn it off entirely. Keep in mind that iOS only allows a 30 second window for the app to refresh before it’s ‘closed’ again. Also iOS is programmed to automatically turn off background refresh for apps that you do not frequently use/open.
Applications that are truly running in the background will display a bar at the top that will let you quickly return to them, this is usually with navigation and calling applications (phone, FaceTime, Messenger Call, etc). My tip and what I personally do is open the backgrounded apps window and scroll all the way to the end and close applications from there, since they are most likely ones that I haven’t visited in a while, and barely use.