App killed from task manager is called foreground state or background state
Apps foreground/background process depends on memory management of your device.

Let's say in your task manager you're running 8 apps and out of 8 you're currently using 3 apps via continuous switching among these 3 apps then your device will see your usage and it could trigger rest 5 apps to run in background for a while but if you don't use those 5 apps for much time then those apps could be triggered to sleep to save power and CPU consumption until you open them again.