AFAIK, Alexa Voice Service and Alexa Skills Kit are only available for device, which is our customized hardware. iOS/Android app, (referred as companion app, created by Amazon), is mainly for settings, such as:
Has Amazon released Alexa API on iOS/Android, instead of device currently?
For the Non-Companion Route (AVS App) , go to AVS App
Just for clarification, There are two different methods to communicate to Alexa.
Companion App
(You have a device (such as a smart speaker) that you want to add Alexa to. So, you build in support for AVS. Great! Now you need a way to authorize it and associate it with the user's account. This is the "companion app" approach. The companion app connects to your smart product and allows the user to login and authorize the speaker to use Alexa and connect to their Amazon account.)
AVS App
(You don't have a device you need to authorize - instead you want to speak to Alexa from within your Android/Iphone application.)