It might be worth starting with the Data Explorer plugin. If you can construct queries on the database, interactively, to get interesting results, you will start to get a picture. You could then run those queries yourself in the Rails console, or (perhaps) using the API. But you would at that point be ahead of me!
For myself, I think getting an understanding of the data model, by way of the database tables, would be the first step. But maybe there’s an API howto which is an easier way in.
(I don’t think you need to understand the technology stack which runs inside the Docker container, for your purposes.)