Top tip(s) for those trying to understand a complex, well-established codebase: start off by rewinding the git history to a very early release.
There's a good chance that most of the core abstractions have survived unscathed, and seeing them surrounded by fewer ossified tendrils will make them easier to make sense of.
As an example, Veloren 0.2 was just 20k lines (compared to 330k today), yet much of 0.2's design principles have survived with surprisingly few changes.