Decidim comes with a powerful API in read-only mode based in the GraphQL specification.
Documentation for this API is auto-generated in each instance of Decidim, usually under the URL:
Accessing that URL will give a full details of all the objects that can be requested and how. It includes the full field types reference.
As an alternative, if you cannot access that URL on your instance you can read about how this work Using the Decidim GraphQL API.