Keep in mind there is a difference between "polishing" and "buffing".

Polishing is when you use some sort of chemical agent, usually in a paste and sometimes combined with fine media, to clean the metal surface and remove oxidation and/or light corrosion. This is usually done by hand.

Buffing is a process that is carried out with power tools, using more aggressive media to actually change the surface finish of the metal product.

As an example, I had some aluminum slip ons for another bike, and they had a grained finish that was applied with sanding abrasives when new. I buffed them to a mirror finish, which they always retained from that day on. Of course, they would still get dirty and oxidized, but this was removed by polishing by hand and in a few minutes the mirror look was back.