I understand that one uses normal baking to bake the normals of a high-resolution (sculpted, in the millions-of-polys range) to an image, which are then set to the normals of the low-resolution (retopologized, in the thousands-of-polys range) mesh to create the illusion of higher resolution, as shown in the following image from Wikipedia:
To save the normal of each vertex to an image, the mesh would have to be UV-unwrapped. However, as I understand it, the entire purpose of retopologizing is to get good topology that can be used for UV-unwrapping. Even if you unwrap the high-resolution mesh, how do you unwrap the low-resolution mesh with the exact same configuration, such that the normals are projected properly?
Or, phrased more succinctly:
What exactly is the workflow for sculpting, retopologizing, and normal-baking?