looking at the installer modules source it looks like you can configure a puppet environment to be directly cloned from git by the installer.
I’m not familiar with the used puppet module for git though, so I might be somewhat off.
The link above leads directly to the relevant lines in the puppet module called by the installer, so you can take a look yourself if you want to
It is indeed a way to deploy modules. The idea is very similar to r10k but implemented when there was no r10k. Nowadays I’d recommend r10k and I’ve actually considered deprecating that git setup. Perhaps I’d slightly hijack this thread if I asked if anyone was still using this feature, but I am wondering