It's such a weird disconnect seeing people that can't go outside of the mindset that everything is a web application.
Systemd ships their documentation (most of what you asked for, actually) with the package, and their environment variables description can be found in /usr/share/doc/systemd/ENVIRONMENT.md (at least on my machine).
When you're dealing with the applications which form the building blocks of a *nix environment your first step should not be google, but grepping through the /usr/share/doc, and using the apropos and man commands.