You write:

There is no easy way to understand how a specific business process is actually implemented. The whole process is defined by how each service reacts to others. There is no central repository, and the implementation details of business processes are scattered everywhere.

Yes, indeed. There is no easy way because the total system architecture is implicitly defined by the interactions of the components.

There is currently no way to actually express the architecture of such a system as runnable/testable and running code. You have the options of either (a) documenting the architecture or (b) leaving it to be defined implicitly.