The developers build the entire framework. The users only have access to the high level, easy-to-use interface. See also Use cases: Excel/GUI vs scripting.
See Framework components and required software for a detailed description of the framework components.
There are two distinct parts in the developer-only lower half of the chart:
The two parts can be developed independently of each other. The interfaces component may require hooks written in other programming languages. However, Python does the driving.