What is the workflow from typing a keyboard key to moving the robot?

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There isn't much to it. Typically the chain is relatively short:

• client reads keys
• client translates key presses into Twist messages and publishes them on the cmd_vel topic
• a motor control (ROS) node receives these messages since it is subscribed to that topic
• the motor control node translates the Twist commands into control signals to the motor drivers

You can inspect this chain using rostopic echo cmd_vel, where you will see what the teleop client (joystick, keyboard, rviz, whichever) is sending.