PX4 Autopilot Trajectory Rather than Waypoint Tracking

https://www.mathworks.com/videos/simulate-and-deploy-uav-applications-with-sil-and-hil-workflows-1653992847786.html

The webinar above shows a pretty good way to design the control algorithm. However, what if we want our trajectory to be generated by ourselves and not from waypoint from something like QGroundControl? How could you for example, fly a specific trajectory that is smooth rather than guided by waypoints? I know in the video circular orbits were flown, but I feel that this was likely enabled by some default functionality in QGroundControl rather than sending a circular trajectory in real time.