Select an object and tinker with it in the left panel of the editor (via Shared - Model - Textures). The extended editor can also help by making some operations more convenient.
I tried leaving null instead of the texture – the mountain is just white, but not transparent.
Then I checked it in maps where it is transparent – the textures are not changed there. Also, if this mountain is placed on another terrain (e.g., dirty grass), it stops being transparent and immediately becomes a regular mountain. It seems that the properties of the object itself have not been changed, but the properties of the terrain texture overlay have. I don't know where I can check this.