  We construct a relative version of the $A_{\rm inf}$-cohomology theory
developed by Bhatt-Morrow-Scholze and relate it to the prismatic theory of
Bhatt-Scholze. The construction relies on the fiber product of topoi. As an
application we show that there is an \'etale comparison of the $q$-crystalline
pushforward after inverting $\mu \in A_{\rm inf}$.
