Try to make map/s. Something visual. Of one or multiple aspects. Cross-reference func/class is easiest but too low level ; needed is crossreference of notions -

Try to make map/s. Something visual. Of one or multiple aspects. Cross-reference func/class is easiest but too low level ; needed is crossreference of notions - in space AND time. Did such thing several times, helps a lot. in my last gig i landed as "owner" of 300K js , in-house event-sourcing-from-scratch.. a complicated mess although somehow organised because of the events. There was a team working on those, but in somewhat darkness - noone knew why x-y-z is so. After some staring at code for a while, found few patterns - some repeated, some once-off, and build a mapper (i am python guy, used Esprima as js parser) to extract and build the graph of domain-event-reason-consequence for all available cases, then made visual maps of those 4 aspects (graphviz). And although noone from the team learned to look/use those, i used them, to guess eventual consequences of some change or answer why this-and-that happened, and shown to various interested parties as "very elevated documentation" :)