Convert MonoGame project to Unity or Implement "Code-only" project in Unity

I have a MonoGame project with builds for Windows, Mac, Linux, iOS and Android.

I want to convert the project (or else simply rebuild it from scratch) in Unity because
Unity appears to have good support for AR/VR and WebGL
which MonoGame does not.

However all my game components are generated in code.
I don’t want to have to manage code that’s scattered around graphic objects
… that will be a major maintenance nightmare.

Is there a way of creating a “code-only” unity project that I implement using Visual Studio?
I went looking for a “Multiplatform Unity 3D project” in the Visual Studio new project dialog,
but I do not see one, even though I definitely have Unity tools for Visual Studio installed.

It is frustrating that no one from Unity3d can be bothered to answer my question.

Am I asking in the wrong place?

I found this: