I’d say the easiest way to achieve this is using ray bounces, assuming that is a topography in Revit. The Data Shapes package has a node for this with a category filter, try it out.
Get the min point of the bounding box of each building, then the ray bounced point and compare their Z values. You can then move each building by a Z vector of that size so it will sit at the topo level. Noting you will only be able to project them to one point of the building this way, so typically this would be centre with some edge points hanging over.