The geometries may be quite unlike our simplified sandwich picture. The n pieces are not always identical - they may have different kinds of doping and maybe even different semiconductors. Since npn transistors are normally operated with the base-emitter junction forward-biased, the construction may be optimized to handle forward current. The emitter may be more heavily doped in order to easily provide electron charge carriers, and the geometry may be devised so that there is a high efficiency for electrons to make it to the collector. Semiconductor designers can do many clever things to optimize devices for particular requirements (high current, fast switching, low noise, ease of fitting inside an integrated circuit, or whatever is important for a particular application). The details of this are beyond what we'll discuss in this course, but here is a bit more info on some particular bipolar transistor types: https://en.wikipedia.org/wiki/Bipolar_junction_transistor.