1. The metallic region and dielectric region are both simply regions in space with either a constant electric potential (metal) or a fixed dielectric constant (dielectric). So the metallic region is not any special metal in particular, it is simply a region in space with a constant potential, similar to what you would have in a "real" device with metal gates.
2. The device from Bulk plugin simply takes you bulk configuration (which is to become the central region in the device) and attaches electrodes in both ends. Therefore, the electrodes have the same crystal structure as the central region has in the ends near the electrodes.