This item under Tips&Tricks explains how to create an in-ground pool:
https://sweethome3d.com/blog/2014/09/04 ... _pool.html

About levels and walls: https://sweethome3d.com/userGuide.jsp#addingLevels
Additionally a basement is a little like an in-ground pool: https://sweethome3d.com/blog/2022/04/21 ... esign.html