Make a Unity project / build with zero scenes, just pure code.

RunmeJohnny.cs

using UnityEngine;

// @kurtdekker - for the purists who don't want ANY scenes in project
//
// All other Unity asset inclusion rules are still in effect, so no
// assets will make it into your project unless they are:
//
//	- correctly in a Resources/ folder
//	- referenced by something in the above
//	- correctly in the Streaming Assets folder / system.
//	- correctly bundled in Asset Bundles or Addressables

public class RunmeJohnny : MonoBehaviour
{
	// This is where it all begins. You will be called here the
	// moment the Unity Engine is up and running and ready for you.
	[RuntimeInitializeOnLoadMethod]
	static void StartWithNoScenes()
	{
		Camera cam = new GameObject("Camera").AddComponent<Camera>();
		cam.transform.position = new Vector3(0, 3, -10);
		cam.orthographic = false;
		cam.fieldOfView = 60.0f;

		// NOTE: the material on this cube will NOT be included
		// in the build because... you didn't reference it anywhere.
		//
		// You will need to get a material on this cube or
		// else it will be hot pink. See comment at top.
		GameObject cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
		// inject an instance of this script to get things started
		var runme = cube.AddComponent<RunmeJohnny>();
		runme.cube = cube;
	}

	// injected by the above (superfluous here, since it is ourselves,
	// but provided as a way to inject other scripts with with stuff)
	GameObject cube;

	private void Update()
	{
		float angle = Time.time * 100.0f;

		cube.transform.rotation = Quaternion.Euler(0, angle, 0);
	}
}