The main difference between self-raising and plain flour is what’s added into the flour, and what isn’t.
Plain flour should only be 100% Wheat flour, nothing more nothing less.
Self-Raising flour will have wheat flour PLUS raising agents in there. This could be baking powder, or cream of tartar and Bi-carbonate of soda.
You can make Self-Raising flour out of plain flour and raising agents at home (you can find out how to do that here) however Self Raising generally shouldn’t be used to substitute plain flour.