How does universal calculator work?
I think you're asking about universal Turing machines? The machine is universal, in the sense that it can compute anything that any other Turing machine can compute.  This is interesting is that Turing machines seem to be a very good model of what can be computed by physical devices, such as actual computers. In particular, we're not aware of any physical computation process (i.e., any mechanism we could use to build a computer) that can compute something that a Turing machine can't. So, in practice, the Turing machine is taken as the definition of what can be computed. (See also the Church&ndash;Turing thesis.)