I think you are confusing the word machine in this context. It is not the usual general purpose word machine that people use in every day life to denote a physi

I think you are confusing the word machine in this context. It is not the usual general purpose word machine that people use in every day life to denote a physical system. The word machine in context of model of computation means a specific type of "formal system" to describe computation. The problem of composition is about how this formal system does or doesn't support it.