re: Slightly perplexed on running apps and killing/ending them.
The individual application lifecycle and system resource management are both significantly different on mobile devices than traditional desktop environments. Application state is not unregulated while using your device; apps are paused, stopped, and resumed in multiple ways, in multiple situations, and typically without visual indication to the user. If you leave an app in the multitasking/recent apps view, and return to it later to find it looks exactly how it did before, it is entirely possible the app was fully stopped while not in use and its Activities (think 'windows' or 'screens') fully 'destroyed' by the system. That's rather unlikely unless you're running low on memory, though. I highly recommend reading through this article by Jerry:
Originally Posted by Dale_M
RAM: What it is, how it's used, and why you shouldn't care | Android Central
The bottom line is that you shouldn't be overly concerned about managing running applications. Background and foreground components of apps are managed separately, but are both influenced by what else you have running, how you're using your device, currently available resources, and your actions in the multitasking/recent apps view. I highly encourage those interested to read up on these topics, as well as to become familiar with what's running on your device and how system and third-party apps behave. Combining such knowledge with task manager applications and battery analysis tools will offer a lot of insight into your device's software routine.