Does the Northbridge Chipset IC have a "core" of some definition
The 'northbridge' chipset forms the interface between the CPU and the outside world.  It connects the CPU local bus with the memory and the PCI/PCIe busses.  The northbridge is responsible for routing memory reads and writes from the CPU to RAM/PCI and from PCI to RAM.  The chip doesn't really have 'cores' in the same sense as a CPU has cores.  Instead, it will have a lot of interface circuitry - high speed serializers and deserializers, PCI express endpoints, etc - as well as FIFOs and routing circuitry.  

Like all silicon dies, the logic would be implemented directly on the surface of the silicon wafer as the logic is implemented with transistors.  There are many metal layers on top of this to interconnect the components, but the transistors themselves are all built on one layer.  

Since the northbridge does not execute software, it will be active immediately following a reset without significant startup delay since no code has to be loaded or booted.  The most significant startup delays would be from high speed transceiver initialization.