What is the round tip on the arm of the MoveIt! ?

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You're asking multiple questions at once. The answer in your post title is about the interactive marker at the tip of the robot arm, which is a tool in Rviz that helps you set goals for MoveIt. It seems like you answered this question by yourself.

The question in your text is about how to set up and control two robot arms left and right, which is more complicated. In short, the best way to do it seems to be:

• Define a URDF with multiple robots in it (read up on how to make your own URDF here. I also recommend looking at xacro macros like this so you don't have to copy-paste your robot files)
• Make a moveit_config for your new robot (the "robot" now contains both of your robot arms) and create a separate planning group for each arm
• Adjust the names of the joints in the SRDF and YAML files so that MoveIt and Gazebo know which of them to plan for and control. Simply changing the names in the URDF does not work, because MoveIt and Gazebo will look for the old joints.

You can find more details in other threads on using multiple arms in moveit, like here, here, or here.