Great question and honestly one that challenges me too. I'm big on the model schemas which INCLUDES the external interfaces. Looks like @andrewtye posted to the master anaplanners. Honestly, I just export the model objects to Excel and use that as a way to ensure I've accounted for all the modules. I either use Lucid Charts or PowerPoint to create the schema. Depends on the size. here's one I did in PowerPoint. Recommend you use the Anaplan colors to be consistent.
It is a common observation "my model is too big for the model map, the model map is so big that I cannot use it anymore".
It is true that the map might be overwhelming at first glance for some models, and it is also true that there is some room for improvement here and few extra functionalities would be very welcomed, such being able to export it (https://community.anaplan.com/t5/Idea-Exchange/Export-Model-Map-to-PDF-Excel/idi-p/34355 but where there is will there is a way! I personally find very useful the fact that we can select a specific module of the map, an then all the related modules will be highlighted whereas the rest will be greyed out:
By doing this (and zooming in and out a few times) I can focus on a specific sub-process of the model and then using tools such PowerPoint, LucidChart (as @JaredDolich mentioned) or https://app.diagrams.net/ (my favourite, thanks @Noemi.andres ) to start replicating and documenting the data flow and the model.