Store Demo mode app ?....

This is known as the Kiosk mode. You can lock an app in the foreground disabling the device's UI and keys. Kiosk mode can be turned on using Mobile Device Management (MDM), Kiosk software or guided access (in iPhone, iPad and iPod touch). With kiosk mode, you can lock any app that is previously installed on the device, or develop an app yourself and set it as kiosk. Setting an app which is not installed on the device can cause unexpected behavior. Some Kiosk software allows you to lock a URL in the foreground, and some provide a custom UI listing the app(s) you can set as kiosk mode.

It's not available to the public. You cannot download it.

You can lock devices in Kiosk mode using MDM or Guided Access. You would have to develop your own app to show in the Kiosk.