Aren't we coming full circle back round to UML? UML is very good at designing and documenting complex code and it's interactions at many different levels, and having a common graphing notation helps ensure everyone is on the same page by looking at the same diagram.

The problem comes when the design and the code get out of step, but there are tools available to round-trip the code and model to keep everything in sync. Executable UML used to be a thing too, but I was never very convinced by it.