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"Since version 10.4 (Tiger), OS X has been moving to a model that supports a 64-bit address space. In this model, called LP64, long integers and pointers are both 8 bytes (64 bits) instead of 4 bytes. In addition, the size_t integer type is 8 bytes instead of 4 bytes. The alignment of these types for LP64 has also increased to 8 bytes. The sizes of all other primitive integer types (char, int, off_t, and so on) remain as they are in the 32-bit model (ILP32), but the alignment of some—namely long long and pos_t—has increased to 8 bytes for LP64."
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Cocoa64BitGuide/MovingTo64Bit/MovingTo64Bit.html
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64-Bit Support
iOS was initially designed to support binary files on devices using a 32-bit architecture. In iOS 7, however, support was introduced for compiling, linking, and debugging binaries on a 64-bit architecture. All system libraries and frameworks are 64-bit ready, meaning that they can be used in both 32-bit and 64-bit apps. When compiled for the 64-bit runtime, apps may run faster because of the availability of extra processor resources in 64-bit mode.
iOS uses the LP64 model that is used by OS X and other 64-bit UNIX systems, which m eans fewer headaches when porting code. For information about the iOS 64-bit runtime and how to write 64-bit apps, see 64-Bit Transition Guide for Cocoa Touch.
https://developer.apple.com/library/content/documentation/Miscellaneous/Conceptual/iPhoneOSTechOverview/CoreOSLayer/CoreOSLayer.html
Uma observação com relação ao comentário do Vitor Pereira é que a referência utilizada por ele trata do OS X e não do iOS, então essa versão 10.4 é do OS X.
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GABARITO CERTO!
O iOS usa o modelo LP64 usado pelo OS X e outros sistemas UNIX de 64 bits, o que significa menos dores de cabeça ao transportar código. Para obter informações sobre o tempo de execução de 64 bits do iOS e como escrever aplicativos de 64 bits, consulte o Guia de transição de 64 bits do Cocoa Touch.
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O sistema operacional iOS suporta o desenvolvimento em 32 bits ou 64 bits.