Product
How to describe a project without a technical spec
You can start with users, actions, data, edge cases and the result you expect. The technical structure can be shaped later.
Start with observable facts
Name the people who will use the system, the actions they perform, the data they exchange and the result they need. Include examples of the current workflow and the points where it breaks.
That is enough to begin discovery. Architecture and technical language should follow the problem, not hide it.