@Colin_Jones welcome to the community and thank you! 
. Okay, so this is a bit of a tough one to explain since there are a few ways to approach modeling topography.
Both ways would work. One thing that is somewhat hidden is the green table can be edited like a regular Arkio geometry, so you could use the table as your landscape starting point and subtract the area in the table to represent your foundation. Personally I prefer the former option, I find it easier to set the lowest point of my site and build up.
Let me know if this helps ! I can make a screen capture of this process if it would 
Also! Don’t forget to say hi 