But I also heard that stainless steel isn’t 100% stainless but merely stain resistant, and that steel is reactive towards other things.
Most “stainless” steel will rust, given the right conditions, like the marine environment, or in a highly abrasive situation … that’s why is is called stain-less.
What makes steel “stainless” is the process of oxygen combining with chromium to form a protective layer on the surface; it doesn’t take a whole lot to disrupt the layer … an abrasive cleaning pad, or carbon steel cooking utensils, for example.
A little oxalic acid, found in several kitchen cleansers, will remove the “stain”.