It’s good you mentioned this specific example. Objects can be setup to share UV space specifically so they can reuse the same material. This optimizes the rendering speed, by reducing draw calls. However, anything using transparency is usually separated from things without, and uses a dedicated material. This is because…
Hey! This is indeed a useful skill, and we do have lots of resources to help explain it (and many experienced artists here who do it!). An atlas is one way to do this kind of workflow, where all the UVs are contained within the 0,1 uv space. You can mirror and stack UVs to reuse space, for example to UV both left and right…