Nodejs回调机制 - 哪个线程处理回调？

I'm new to nodeJS and was wondering about the single-instance model of Node. In a simple nodeJs application, when some blocking operation is asynchronously handled with callbacks, does the main thread running the nodeJs handle the callback as well?. If the request is to get some data off the database, and there are 100s of concurrent users, and each db operation takes couple of seconds, when the callback is finally fired (for each of the connections), is the main thread accepting these requests used for executing the callback as well? If so, how does nodeJs scale and how does it respond so fast?.I'm new to nodeJS and was wondering about the s