On functor double $\infty$-categories

In this paper, we study double $\infty$-categories of double functors. To this end, we exhibit the cartesian closed structure of the $\infty$-category of double $\infty$-categories and various localizations. We prove a theorem that characterizes the companions and conjoints in functor double $\infty$-categories via the notion of companionable and conjointable 2-cells in double $\infty$-categories. Moreover, we show that under suitable conditions, functor double $\infty$-categories are horizontally closed. Throughout the paper, we highlight a few applications to $(\infty,2)$-category theory and indexed exponentiability.