Android has the largest installed base of all operating systems (OS) of any kind. Android has been the best selling OS on tablets since 2013, and on smartphones it is dominant by any metric.
Since Android devices are usually battery-powered, Android is designed to manage processes to keep power consumption at a minimum. When an application is not in use the system suspends its operation so that, while available for immediate use rather than closed, it does not use battery power or CPU resources.
Android manages the applications stored in memory automatically: when memory is low, the system will begin invisibly and automatically closing inactive processes, starting with those that have been inactive for longest.