The opposite of swagger-core?

I am an experienced technical writer but new to the management of Swagger or OpenAPI.

We currently use Swagger-Core, where all the settings are in @Operation or @Parameter within Java or Python files. 

However, I have seen set ups where those configuration statements are in yaml files. The biggest advantage is that the text inside a description tag can get as extensive as needed. 

Somehow they are connected to the Java or Python files.

What is that system called? How does that get done?