I see everywhere the etale sheaf $\mu^{\otimes d}$, but I cannot see the precise definition.
Concretely, what is the sheaf $\mu_3 \otimes \mu_3$ over ${\rm Spec}\mathbb{Q}$, for example described as ${\rm Gal}(\mathbb{Q}^{sep}/\mathbb{Q})$ module?
I know that tensor of sheaf ${\cal F} \otimes {\cal G}$ is defined by the sheafification of the presheaf $U \to {\cal F}(U) \otimes {\cal G}(U)$ (over some ring sheaf), but I don't know $\mu(U)$ is module over what ring.
Here I recognize for example $\mu_3({\rm Spec} \mathbb{Q}(\sqrt{-3}))$ is cyclic group of order 3.