Where are parallel execution threads carried out in the system?

Parallel execution streams in iOS are created and managed at the operating system level, using multithreading mechanisms such as POSIX Threads (pthread) and Grand Central Dispatch (GCD).

• Grand Central Dispatch (GCD) is a high-level API that allows creating and managing task queues that can execute in parallel on different threads. GCD automatically distributes tasks across available CPU cores.

• NSThread is a lower-level class for creating and managing threads, but in modern applications, GCD or OperationQueue are more commonly used.

Thus, parallel execution occurs at the system thread level, which is managed by the iOS kernel, and the developer interacts with them through high-level APIs.