Manipulating Terrain in Unity via script in Run Time

I need someone to make me a simple example how to place an object on terrain in run time,( in my case this object is a trap door prefab leading downstairs ) then flatten that area of terrain, remove grass and trees underneath trapdoor, and then make/paint a hole in the area of the prefabs actual hole! this obviously has to be done in latest 2019 unity , because paint hole is only available in latest unity version! I am aware that this is probably ridiculously easy thing to do for anyone who has knowledge, and I have tried using it myself in edit and it works great... but I have no knowledge to do this via script, I have never used terrain before and don't have time to do research on this! someone said API for this is Sethole() but I could not get it work ... anyway I would prefer for someone to show me how is this done ... I would provide a prefab of the trap door and all you would need to do is to make a simple project placing trapdoor onto a terrain with a click of the mouse and wherever trap door are placed flatten terrain remove grass and paint a hole onto terrain in the obvious trap door location of the hole! also include delete button .... Because if trapdoor prefab has been destroyed terrain hole paint mask also has to be removed! I will require unity project 2019.3.6 sent with all scripts to make sure this works as expected I myself can make a simple cube placer/ in my case trapdoor on a terrain in 10 min... but I have no knowledge of Terrain APi to add a mask to flatten and make a hole.... this probably takes extra 5 min for someone who knows how to program ! I cant take seriously anyone who think this takes more then a day!