The simple answer is that the OS does it automatically for you. Apple does not want the user to bother about app data and cache and want the user experience to be good.
The technical answer:
First you have to understand the prime difference between how the OS works. Android apps can keep working in background and keep accessing data, download stuff, activate in background from a push and much more. Whereas in iOS, it's limited and work in background is only via user permissions.
Secondly, when the app goes to background, it can make requests only for 30 seconds.
Also, when the app is in background, it is not consuming memory or cache data. Apple have themselves said that terminating the app or putting it in background makes no difference.
Let's say there is an app which is consuming a lot of memory in background (VOIP, music apps can do and have permissions). Now if you are playing a game which requires more memory, the OS will kick in and kill the VOIP/music app to give the current game more memory to work smoothly.
Although the above scenario does not happen in 99% of the times today because there are a lot of resources available, a lot. The A9 chip in iPhone 6s, along with 2GB ram is quite enough for whatever is available right now, given how Apple manages the OS. You'd be surprised to know that most heavy/intensive apps takes about 50-100 mb when they are running which is low given there's 2gb available.
This is where iOS is intelligent than Android. It does manage the system better in a lot of ways.