For any application using Phoenix, the web is a delivery mechanism and not a Domain.
i.e. a todolist app might have two domains Tasks & UserManagement, they are both available over the web but there is no web domain.
Within Phoenix the web can be considered a domain but that would be an argument for how the Phoenix project might order its code and should not be a concern of the end user.
I made some similar comments in my recent talk on the MVC architecture. This is the resource I use to explain my reasoning is Cohesion (computer science) - Wikipedia
Grouping things because they are “controllers” is Logical cohesion and according to that page that is the second “worst” type of cohesion.
