I don’t think there is a guide.
Some sections of the source are clean and nice (and new), others can look ancient and have a different, uglier style (NPC code unfortunately is often the latter). As a result, it would be a bit tough to have an updated guide. There’s ‘docs’ directory, but that’s mostly for jsons.
Plus, the code actually evolves quite fast. Like when BevapDin changed the item class - that was pretty much a total rewrite of how items were represented internally that removed a lot of bad practices in code (lots of type casting) and enabled multi-type items. Keeping up with changes like that would require a dedicated docs writer.
It’s not that tough to get used to DDA code. As long as you’re not dealing with coordinate conversion (submaps, overmaps, local map) - this one takes a while to understand.