Everything here is my opinion. I do not speak for your employer.
A git-subtree tutorial
Jakub Suder has a nice tutorial on how to use my git-subtree tool to manage git repositories that track other projects in subdirs.
It's quite nicely written, and unlike my own documentation, it has pretty diagrams.
My latest project is Tailscale: the easiest way to use WireGuard and 2FA.