How can I transfer my in-app purchases to my other Apple devices?

Please make sure that your other Apple devices are signed in with the same Apple ID that made the initial purchases. Then install the app and press the “Restore Purchases” button, that is mainly located on the main menu (though this may vary depending on the app).
It’s important to note that in-app purchases can be restored or transferred only within the same store where they have been performed, as each application store has its own billing and account systems.