Are you aware that the gz-sim repository contains a lot of demo worlds?
This is one of the primary sources of information on what exists in Gazebo Sim and how to use it.
The system plugins can be found here. Their header files typically contain documentation, for example see here for the UserCommands plugin.
For ROS 2, there are similar repositories: the examples repository and the demos.
See also this previous post where I shared some things I learned wrt. finding info on ROS / Gazebo.