What is the best workflow for decimating part of mesh on huge aerial models with high poly count? I'm trying to decimate using sculpting on a parts of the mesh, the model is quite big, about 40 million polys. I'm fairly new to blender and i would like to get some pointers to tutorials or tips on how to keep my textures intact but decimate parts that are of no interest, for example trees and some of the terrain. 

I have a textured model from Metashape/RC, and i want to preserve the UV-map. When using the decimate modifier i get a good result without having to unwrap and bake, but if i want to keep the high detail on a single building this doesn't seem like the right approach. 

I'm sure someone has a great workflow for this and wouldn't mind sharing it. Coming from Metashape and Reality Capture, blender can be a bit overwhelming so any thoughts would be appreciated :)

Cheers
In Sculpt mode, you could try to use the Dyntopo > Collapse Edges mode. It will change the minimum geometry size to the threshold chosen in Detail Size. Make sure the brush is at a Strength of 0.