iOS - Restricting device to work with a single app only
The desired functionality is called Single App Mode, and is available for supervised devices.

An iOS device can be setup for Single App Mode using a Mobile Device Management (MDM) software.

A few articles shedding light on this topic are quoted below:

How to use iOS Single App Mode


  Single App Mode (SAM), often referred to as “Single App Lock” is a feature for supervised devices that restricts the device to running only one app. While Single App Mode is enabled, the selected app will stay in the foreground.


Set up your iPad to give tests and assessments


  Autonomous Single App Mode is a feature of assessment apps that locks iPad in to the app for testing. If your app doesn’t support Automatic Assessment Configuration, you can set up your iPad for Autonomous Single App Mode using Mobile Device Management (MDM) software. Your MDM software installs a profile that identifies your testing app on the device.


How to Setup Kiosk Mode &amp; Lock Your iPad to Just One App – Part 2


  In iOS version 9, Apple introduced “Supervised Mode” which enables a feature called “Single App Mode” on your iOS device. Single App Mode is a feature for supervised devices that restricts the device to running only one app. While Single App Mode is enabled the selected app will stay in the foreground, even after the device is rebooted.