Why OLED/AMOLED screen technology create ilussion colour?
For other color, like YELLOW it combined from RED and GREEN. But, is it really combined? I mean it combined by special glass that each emitting light through it then combine them and create new light (yellow) .

Assuming the subpixels in the display are too small to resolve by eye (meaning that you cannot see them), then actually they are combined by the lens of your eye and only form yellow as they hit your retina.
If you have a very low resolution display and look closely, you will see the individual colors.  In this case, they're large enough that your eye can resolve them, so they do not mix to form secondary colors such as yellow.