Info: Quake C and engine resource links - Subject9x/battleMETAL GitHub Wiki
The following are a list of helpful links and current engines for modding Quake today. The links were invaluable to me as I developed battleMETAL and they will certainly help you when digging into the code for battleMETAL.
Quake Mapping discord https://discord.com/invite/j5xh8QT
Darkplaces Engine discord https://discord.com/ZHT9QeW
http://www.gamers.org/dEngine/quake/spec/quake-spec34/qc-menu.htm
- short overview of the Quake C language specification along with the stock, original engine builtins.
- their forum was recently broken and they're still trying to repair it, but its the best place for knowledge.
http://www.insideqc.com/qctut/
- this gives you a great look at what is quick to do with the engine.
these are the most popular forks of the Quake 1 engine
(Most modding of Quake engines is for Quake 1 and using these forks)
FTE Quake - FTE
Quakespasm - Quakespasm
Darkplaces - Darkplaces
Specific Files /dp/dpextensions.qc /dp/csextensions.qc
- both of these files have documentation in them that mostly describes how to use the Darkplaces engine extensions.