In the 1990's, symmetric monoidal categories of spectra were introduced, That raised the question of how one can construct rings, modules, and other multiplicative structures in such categories. I will explain a new approach that starts from an elementary new understanding of three equivalent formulations of exactly what symmetric monoidal categories are: the traditional definition made precise by Saunders MacLane long before my time, a folklore operadic definition as pseudoalgebras over the permutativity operad that was understood in the 1970s, and a new definition as special pseudofunctors from the category of finite based sets to categories. With this equivalence, it is quite easy to construct a functor from the multicategory of symmetric monoidal categories to the multicategory of spectra. The punchline is that this approach works equivariantly for any finite group G. For example, that gives an easy new way to construct the algebraic $K$-theory of a Galois extension with group $G$ as the homotopy groups of an $E_{\infty}$ ring $G$-spectrum.