  A universal Turing machine is a powerful concept - a single device can
compute any function that is computable. A universal spin model, similarly, is
a class of physical systems whose low energy behavior simulates that of any
spin system. Our categorical framework for universality (arXiv:2307.06851)
captures these and other examples of universality as instances. In this
article, we present an accessible account thereof with a focus on its basic
ingredients and ways to use it. Specifically, we show how to identify necessary
conditions for universality, compare types of universality within each
instance, and establish that universality and negation give rise to
unreachability (such as uncomputability).
