You can build, edit, and texture custom geometry in Unity with the actions and tools available in the ProBuilder package. You can also use ProBuilder to help with in-scene level design, prototyping, collision Meshes, and play-testing.
ProBuilder also comes with a Scripting API, so that you can write C# scripts to make your own tools and customizations.
Some of the advanced features include:
In addition, the ProBuilder package includes a Model export feature, which you can use to tweak your levels in any 3D modeling software.