How to extrude in script?

[soft wing]

If you mean editing the mesh in Unity, that would require mesh generation - if you mean just making a object taller from its base, you could parent the object and move the base to the parents zero/center, then change the y scale of the parent, though the former is probably much easier to do in Blender and export out to Unity, you can also import a whole Blend file and work from that file to help with making many quick iterations, why might you want to do this by script?

[alpine mesa]

something procedural (block walls)