  We show how to calculate the relative tensor product of bimodule categories
(not necessarily invertible) using ladder string diagrams. As an illustrative
example, we compute the Brauer-Picard ring for the fusion category
$\operatorname{Vec}(\mathbb{Z}/p\mathbb{Z})$. Moreover, we provide a physical
interpretation of all indecomposable bimodule categories in terms of domain
walls in the associated topological phase. We show how this interpretation can
be used to compute the Brauer-Picard ring from a physical perspective.
