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I have found the answers for most of my questions and now I'm able to do teleoperation. I am writing the answers in case someone else might the same problems.

So the answers;

1. In the odometry transform broadcaster, my mistake was to only create transforms between /odom and /base_link. This was the reason why I the robot only moved around a circle of odom frame. I have fixed this.

2. I still don't know why I only see /odom frame and no higher world frame.

3. Joint_state.velocity[] acts as what we treated. It's just a variable. So in my case since the joints are continuous they should be taken as angular velocities.

4. I didn't have a time differential so my usage was wrong. It is fixed by;

   while (ros::ok())
   {
   ros::spinOnce(); //check new messages
   current_time = ros::Time::now(); //set current time
   
   nh_.getParam("wLeft",wLeft); //get left wheel angular velocity value from parameter server
   nh_.getParam("wRight",wRight); //get right wheel angular velocity value from parameter server
   
   vLeft = wheelRadius*wLeft; //linear velocity of left wheel
   vRight = wheelRadius*wRight; //linear velocity of rigth wheel
   vx = (vRight+vLeft)/2; //linear velocity in ROBOT frame
   wth = (vRight-vLeft)/length; //angular velocity of the ROBOT
   dt = (current_time - prev_time).toSec(); //time differential
   delta_x = vx*cos(th)*dt; //change in x axis in world frame
   delta_y = vx*sin(th)*dt; //change in y axis in world frame
   delta_th = wth*dt; //change in the orientation
   
   x += delta_x;
   y += delta_y;
   th += delta_th;
       //broadcast transforms
       broadcastTF(&broadcaster,"base_link",x,y,0,th);
       broadcastTF(&broadcaster,"wheel_left",x,y,0,th);
       broadcastTF(&broadcaster,"wheel_right",x,y,0,th);
       //set previous time
   prev_time = current_time;
       //rest until loop rate is done
       loop_rate.sleep();}//end of while