After reading the documentation on Ktor Modules, I'm unclear on what advantage there is (if any) to separating a Ktor Server into different 'modules' vs just breaking down a single module into sub-functions for organisation. In particular; it's not stated whether installing a Plugin is scoped to the current module or global to the Application.

A plugin installation is global to the Application.

Is it really? Why is it specified in a module then

In my understanding module is just an extension function that you can point to in a HOCON config or call directly in tests.