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At a low level, in iOS systems, locks are implemented using several main mechanisms:

• Mutexes (mutual exclusion) — ensure mutual exclusion, allowing only one thread to own a resource at a time.
• Semaphores — allow limiting the number of threads that can access a resource simultaneously.
• Spinlocks — locks that actively wait for a resource to be released, without going into sleep state.
• Atomic operations — operations that are performed atomically without interruptions, often used to implement lightweight locks.

In iOS, these mechanisms are implemented at the POSIX level (e.g., pthread_mutex_t) and through low-level APIs, as well as via high-level constructs like GCD (Grand Central Dispatch) and NSLock, which utilize these mechanisms under the hood.