There's no proper iOS emulator. The closest thing that exists nowadays is
Heilei, which doesn't support Jailbreak and IPA installation is only allowed for developers.
In order to mod iOS games you need a jailbroken iOS device with the cydia packages: Filza, Cracker XI, ApplicationLoader (I'm not sure if this is the correct name) and Theos.
Procedure:
> Cracker XI to decript the IPA.
> Use filza to transfer the decrypted IPA to your computer
For lib games:
> Classic modding procedures, find offsets and opcodes to place there (same as android, Il2cppDumper, IDA...)
> Create your mod from scratch or use an already existing template with your offsets and opcodes (using theos)
> Compile your .deb mod, install and test. (using theos)
> If works, feel free to share this .deb with other users