You probably want that one in config.exs.

You often have these files:

config/
- config.exs
- dev.exs
- prod.exs
- runtime.exs
- test.exs

config.exs is your base config and others are imported afterwards, depending on the environment, like so:

# This file is responsible for configuring your application
# and its dependencies with the aid of the Config module.
#
# This configuration file is loaded before any dependency and
# is restricted to this project.

# General application configuration
import Config

... your config here ...

# Import environment specific config. This must remain at the bottom
# of this file so it overrides the configuration defined above.
import_config "#{config_env()}.exs"

If you’re using releases, runtime.exs will end up in the release directory, so you can edit it after the fact. The others will vanish, like magic (or be compiled).

All the config will end up in your Application env, which you can retrieve using Application.fetch_env/get_env and friends. Notice the typespec for Application.put_env is put_env(app(), key(), value(), timeout: timeout(), persistent: boolean()) :: :ok, where value() :: term().

Config.config/3 is config(root_key, key, opts). It talks about keyword lists, which may lead us to think we can only put keyword lists in there, but as we can see from our config files, and Application.put_env, we can jam just about anything in there. Typically it is either a single value or a keyword list.