I dont know much game development, just an observer but: tremulous is an ioquake3 mod. I guess there are tutorials about ioquake3 out there but the truth it is outdated and adhoc technology resulting from a specific moment in game history.
I use to play Urban Terror (excellent game dynamics, really) nowadays and it uses ioquake3 too. I dont know how they monetize but they know their trade and are actually abandoning that technology in favour of "Unreal Engine" which is more modern and has much better documentation.
Look this anecdotic post from UrbanTerror devs
http://www.urbanterror.info/news/530-the-story-of-urban-terror-hd/