How exactly this Proportional controller graph is working, what happened after one overshoot , why it didn't overshoot again after one overshoot ?

 

1. The first overshoot is because of the higher initial "error" [setpoint - process variable]
2. Subsequently, the error amplitude got minimized and hence no aggressive behavior of controller

 

Check out this article about PID's, hopefully this will help. https://variablefrequencydrive.com/how-to-make-pid-loops-work-for-you/