This repo is the practical examples to go with this presentation about XcodeGen.
It uses tags to progress from one step to the next. So once you have the repo cloned locally please checkout step-1 tag and start from there.
git checkout tags/step-1
You can also take a look at the pull requests. Here you can see a diff between each step and compare what a code review would look like between the .xcodeproj default method vs the XcodeGen method.
You can also filter the PRs for those that used the xcodeproj method or those that used the XcodeGen method.
Please look to the help page for more information.