why rgb

Why do they use rgb for various color screens when green is not a primary but the combination of yellow and blue light. Sorry for my ignorance.

Maybe electrons don't emit yellow light?

It would make the size of the electron gun small with an RGB combination whilst still being able to display all the colors and tones. Well I don't know for sure, but that sounds to me to be the most likely answer :)

Red green and blue are the primary colours when dealing with light, i.e. additive primaries. Arrange three lamps, one each red green and blue, and arrange them to shine on a white screen. Arrange them so that they overlap each other partially and completely. Where the red and green overlap, you will see yellow, where the green and blue overlap, cyan, and where the red and blue overlap, magenta. Where all three overlap, you will see white.

Yellow + blue = green works for subtractive primaries for example, when mixing paint.