Some fundamental doubts about DisptachQueue and GCD

I understand that GCD and it's underlying implementations have evolved over time. And many things have not been shared explicitly in Apple documentation.

The most concepts of DispatchQueue (serial and concurrent queues), DispatchQoS, target queue and system provided queues: main and globals etc.

I have some doubts & questions to clarify:

1. [Main Dispatch Queue] [Link] Because the main queue doesn't behave entirely like a regular serial queue, it may have unwanted side-effects when used in processes that are not UI apps (daemons). For such processes, the main queue should be avoided. What does it mean? Can you elaborate?
2. [Global Concurrent Dispatch Queues] Are they global to a process or across processes on a device. I believe it is the first case but just wanted to be sure.
3. [Global Concurrent Dispatch Queues] Does system create 4 (for each QoS) * 2 (over-commiting and non-overcommiting queues) = 8 queues in all. When does which type of queue comes into play?
4. [Custom Queue][Target Queue concept] [swift-corelibs-libdispatch/man/dispatch_queue_create.3] QUOTE The default target queue of all dispatch objects created by the application is the default priority global concurrent queue. UNQUOTE Is this stil true?

   • We could not find a mention of this in any latest official apple documentation (though some old forum threads (one more) and github code documentation indicate the same).

   • The official documentation only says:

     • [dispatch_set_target_queue] QUOTE If you want the system to provide a queue that is appropriate for the current object UNQUOTE
     • [dispatch_queue_create_with_target] QUOTE Specify DISPATCH_TARGET_QUEUE_DEFAULT to set the target queue to the default type for the current dispatch queue.UNQUOTE
     • [Dispatch>DispatchQueue>init] QUOTE Specify DISPATCH_TARGET_QUEUE_DEFAULT if you want the system to provide a queue that is appropriate for the current object. UNQUOTE

   • What is the difference between passing target queue as 'nil' vs 'DISPATCH_TARGET_QUEUE_DEFAULT' to DispatchQueue init?

5. [Custom Queue][Target Queue concept] [dispatch_set_target_queue] QUOTE The system doesn't allocate threads to the dispatch queue if it has a target queue, unless that target queue is a global concurrent queue. UNQUOTE
   • The system does allocate threads to the custom dispatch queues that have global concurrent queue as the default target.
   • What does that mean? Why does targetting to global concurrent queues mean in that case?
6. [System / GCD Thread Pool] that excutes work items from DispatchQueue: Is this thread pool per queue? or across queues per process? or across processes per device?