Surface cutting

I want to cut the model along a specific boundary.

1. Create a surface using the Region.CreatePolygon() function.
2. Convert to Mesh using Mesh.ConvertToMesh() function
3. Extrude with Mesh.ExtrudePlanar() function

I created a model like the one shown in the picture using the same process as above.
However, I only need the model above the yellow face.
I want to remove the area below the yellow face. I've tried several things, but it's not working.
I'm currently using the brep.CutBy() function, but it's not working.
If there's a way to remove the area below the yellow face, please let me know. Or, if there's a way to extrude only up to the yellow face, please let me know.

For reference, I will briefly explain the method I tried.
1. UtilityEx.ConvexHull(), Surface.ConvexHull()
2. Surface.Loft(), Brep.Ruled()
3. Brep.Difference(), Brep.Intersection()