How does Gazebo publish ROS messages?

edit

HI does someone mind please explaining how exactly Gazebo is able to publish ROS topics on a high level?

My understanding is that Gazebo uses Google Protobuff and boost::ASIO to publish/subscribe topics between its own libraries (e.g. physics, rendering); however, how are these messages "converted" to ROS messages? My guess is that there is a Gazebo plugin to access one of the Gazebo libraries and in that plugin there is code to convert a Gazebo message to ROS message, initialise a ROS publisher, and then publish the ROS message, but can someone please confirm?

Thank you for your help in advance!