Is it required to set the EKF origin when flying with px4flow with PX4 stack or any non gps navigation device like in Ardupilot , if yes then how to do it through QGC

In non gps navigation if you want to move locally (e.g. with an RC) you won’t necessarily need to set an EKF origin yourself. However, if your plan is to do offboard missions an EKF origin is required, but I don’t recommend using only optical flow for that.