Sometimes you want to synchronize Entities that are connected to other Entities. These relationships can be references between Entities, but they can also involve direct parent-child relationship between game objects, or more nuanced use cases.
Here's a guide for what technique to use, depending on the situation:
If you have an Entity that needs to keep a nullable reference to another Entity, use a normal Entity reference. This includes any existing MonoBehaviour that has GameObject or Transform fields that you want to synchronize over the network.