Unity3d ScriptableObject Reference Example, concept from https://youtu.be/raQ3iHhE_Kk

ScriptableObjectReferenceExample.cs

/*

 GameObject A would have PlayerControler and SetPlayerReference.
 GameObject B would have WantsToUsePlayer.
 
 SetPlayerReference and WantsToUsePlayer have same PlayerReference set via the inspector.
 
 WantsToUsePlayer will be able to use PlayerReference to have reference to PlayerControler. Even if GameObject A is spawned from a prefab after the game starts.

*/
[CreateAssetMenu()]
public class PlayerReference : ScriptableObject
{
    public PlayerControler player;
}

[RequireComponent(typeof(PlayerControler))]
public class SetPlayerReference : MonoBehaviour
{
    public PlayerReference reference;
    public void OnEnable()
    {
        reference.player = GetComponent<PlayerControler>();
    }
}

public class PlayerControler : MonoBehaviour
{
    public float health;
}

public class WantsToUsePlayer : MonoBehaviour
{
    public PlayerReference playerReference;

    public void Update()
    {
        var health = playerReference.player.health;
        // does something with health
    }
}