"Artists traditionally" is doing a bit of heavy lifting here. Neither artists nor art teachers explain a concept that all colours can be mixed from RBY primaries for the simple reason that the jig is up the moment they start painting. Rather artists very loosely describe these colours as primaries as a starting point to the concept of mixing colour.

In actual art classes they teach the colour wheel, which usually leads to around 6 colours being used for general painting in addition to white and other colours to darken. They're also not super fixated on what these colours should be specifically instead using terms such as "warm red and cool red", "warm blue and cool blue" etc. There is always a constant recognition that these will still present a limited gamut and that may be a deliberate artistic choice.

There is also an understanding that additional paints may be needed for getting a stronger colour or simply the convenience of not having to mix so much. This thinking continues through into professional printing hence the Pantone system or more interestingly with certain Epson printers that can include ink tanks specifically for violet, green, orange (amongst many others) - where the software will do the heavy lifting of blending these colours with the CMY model to produce a vivid gamut.

On the topic of "Key". Key is the pigment used to introduce detail and tone. For a two colour artwork the Key could simply be the darker of the two shades. We know it as black, but in some print processes it's an imperfect black-like colour that blends well with CMY to produce detail. Again in those Epson printers mentioned earlier, multiple blacks are used depending if it is to be used for creating detail in the coloured areas or simply as a block of black.