You can create and explore in High Fidelity using external software for scripting and 3D Modeling. We've compiled a list of software options you can use and you're free to use these or any other tools you prefer.
This is a list of script editors you can use to write and run scripts in High Fidelity.
We also have a list of software you can use for your modeling needs. Just ensure that they can create or convert your models to FBX.