It would be much clearer if you indicated the exact time.
It is in the begining of the video, uploaded to You tube, approximately between seonds 13 and 18.
I only showed that you delete the surplus, but I didn’t keep the entire perimeter how it is deleted because it is identical, you select what you want to delete and press Delete. If you want it to be a straight line, you can draw one on top and with the Drape command you project it over the terrain and when you delete, you delete the geometry up to it.
Greta, Thanks for the instrcution