Each mobile version (iOS, Android, and Amazon) is a separate purchase from each other, and from the PC/Mac version, due to App Store requirements and other factors. So:
If you purchase the game for computer (on wolfquest.org or itch.io or Steam), you can download it for Windows or Mac OSes.
If you purchase it from the Apple App Store for iOS, you can download it to any iOS device (iPhone or iPad) that has your Apple ID.
If you purchase it from the Google Play Store, you can download it to any Android device that has the Google Play Store app with your Google ID.
If you purchase it from the Amazon Appstore, you can download it to any Kindle or Android device that has the Amazon Appstore app with your Amazon ID.
Then, after creating an account, you can log into the game on any of the above devices and sync up your data.