The iOS code is substantially more complicated to get working compared to the other platforms. You will need an reasonably modern Apple computer with Android Studio, and Xcode.
I will write a more full guide here in the future, but for now a great starting point is: Deploying your libGDX game to iOS in 2020. Some steps can be skipped as the actual application code is all done for you. Provisioning profiles and Xcode setup are the largest hurdles.
Consult the other platform guides for how to get the code onto your machine. Additionally, please contact me if you wish to make a version based on Shattered available on the App Store.