Q: closing all apps with one command

There is no method to do so, nor is there a reason.

There is normally no need to quit apps. The iPhone handles memory. but if you need or want to quit all apps this is what you have to do:

To quit an app double click the Home button to reveal the app switcher. Flick up on the page preview of all the apps you want to quit. The page preview will fly away and disappear. That quits the app. 

You might want to read this article for some background on multitasking:

Understand multitasking and background activity on your iPhone, iPad, or iPod touch - Apple Support

This article is also pertinent:

http://www.howtogeek.com/204552/no-closing-background-apps-on-your-iphone-or-ipa d-wont-make-it-faster/