I think what you wrote here is just straightforwardly not correct, eg. you can absolutely set a variable and multiply arbitrary numbers in addition to working w

I think what you wrote here is just straightforwardly not correct, eg. you can absolutely set a variable and multiply arbitrary numbers in addition to working with vertex data. There are also a lot of other ways to store and interact with data, eg. textures (either procedural or painted). Maybe you should have another look.