when I run GlobalScope.launch, is it backed by a threadpool? or it will run in the main thread?

It runs in a thread-pool of

Dispatchers.Default

are all coroutines running in some kind thread pool? or I can launch them in the main thread?

You can explicitly specify dispatcher. You can read more here http://kotlinlang.org/docs/reference/coroutines/coroutine-context-and-dispatchers.html