It's a really thin, tiny layer. What it does is help to prevent excess copper leaching. Perfectly harmless and desirable. If you were to soak in a concentrated vinegar or other acid, the copper would pit... or would otherwise get worn down to the raw metal. And when it does that, instead of copper oxide, the thin layer has various copper salts that are not as good at maintaining the integrity of the copper. That's what I meant by no harsh treatments with chemicals. Maybe as a one-time reconditioning process going more harsh is fine, but generally, a light acidic scrub and it's good to go.
With harsher chemicals, pitting is also a concern. One of the beautiful aspects of your approach with the 3M pads is that they are ever so slightly abrasive. And if you are really thorough in the cleaning, going in a repeat pattern, in the same direction with the pads, it puts an excellent finish on the copper and has this "sanding" effect. Helps to keep a smooth surface, which prevents biofilm from forming and removes pits and hidey holes that bacteria live in.