What type of queue is used in default standard implementations?

In standard iOS implementations, a Serial Dispatch Queue is used by default for task execution. For example, the main thread (main queue) is a serial queue that executes tasks one by one, in the order they are added.

For multitasking and parallel execution, a Concurrent Dispatch Queue is used, but it is not the default queue.

Therefore, if you create a queue without specifying the type, it is most often a serial queue.