By default config/config.exs is not generated because it was wrongly used in Elixir libraries. Please look that Elixir is not working like a framework which forces many things like for example file names. You can freely define MyModule module inside some_module.ex file. The same goes to using or not config.

It’s up to you if you decide to import Config (which is recommend way now as Mix.Config is soft-deprecated).

This means that the real question is:

Is config/config.exs a good solution for my use case?

A typical answer for this question is:

Yes, as long as your application is not a library.

You may decide to not use a Config when for example you want to store the changes of configuration made at runtime, but such situations are called edge-cases. For a typical application it’s ok to use config/config.exs as in books you’ve read.

If you know that something have changed since x release then it’s worth to check it’s announcement.

Here are official sources:

Thank you. I had read that.

I suppose I’m just still confused by it. I’m not sure how to translate these notes into “Okay, this code goes here now not there.”