It's not hard if you understand how cvs works. The email says the files are in the "src" module, so you select that from the cvsweb page. Then it's just a matter of drilling down to the individual files. Here is the diff for the Makefile for example:

https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/games/quiz/Mak...

As for how you work like that, the number of people with commit access is small and they all know each other. There aren't a lot of branches. Most of the collaboration features of git aren't needed.