Would be nice to have a short summary or detailed answer on this.

Some resources to discuss this topic:
https://dlang.org/spec/class.html
https://dlang.org/spec/module.html

You can only have one instance of a module, so it is really more a collection of types, functions, etc. that help each other do a particular job.

Then structs and classes can have multiple objects of each type so you use them to do most the stuff you run. You can make class objects in a loop or array, you can make temporary ones, replacable ones, etc. Modules cannot do any of that.

So in short:

* use a module to group classes and functions
* use a class when you need to subclass it and customize functionality and/or need several instances
* use a struct for most other cases, it is a generic collection of data