The GitHub repos are current, you just have to find the branch the current work is being done on. Also, the model part was split off from the main Fieldworks repo a few years ago.
In my experience, those files are a helpful reference, but it can still be pretty hard to figure out exactly what you're looking for unless you spend a good amount of time familiarizing yourself with and searching the code base.
It's my understanding that the xml model file is used to automatically produce a good amount model code base, but there are lots of other additions that are done "manually".