There are many concurrent approaches to organize application configuration nowadays.

Classic .ini files, .json, .toml, .yaml, configs, modern .env and, of course, container environment. And don’t forget about CLI arguments! Am I missing something?

Let me be honest, I really dislike any implicitness in interfaces. Same for the CLI of course. Any of your interfaces, whether public or internal, API or object interface, a class method or module facade – they have to cooperate fair. The contract between you and the other side should be explicit, rightful and without any notes in small text at the bottom of the page.