No ROS topic between nodes in rqt_graph

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I am a beginner in ROS, and I am working on a quadcopter project using ROS and Gazebo. To give you a bit of the background, here is what I did:

1. I created a quadcopter CAD model in solidworks.
2. Then I exported it as an urdf file to ROS and Gazebo.

Now my next task to move the quadcopter using python code. But when I type commands like rostopic, I don't see any valuable topic to control my quadcopter. I want the propellers to rotate so that the quadcopter can move.

In the rqt_graph, I don't see any topic between nodes.

How do I tackle it?.