Curvy Shapes in Unity Terrain editor?

I’m a beginner to Unity. I’m looking for a way to draw some curved shapes like a arc roads/streets(semi-circle perhaps?) or other detailed things. The problem is, i can’t achieve them with the unity in-built terrain tools/brushes since they are just brushes. Is there any other free terrain editor or something or free unity plugins? Any kind of help would be appreciated.

Create the height map in Unity, then export it as a raw bitmap. Import that into Photoshop or whatever paint package you have purchased, and make your changes there. Save it, and then re-import into Unity.