The MonoBridge is a system that makes sure every GameObject is linked to its networked representation. It essentially interfaces between the GameObject world and the DOTS/ECS-based coherence SDK code running "under the hood".
Note: All of this code is accessible to you and can be interfaced with directly using DOTS.
When you place a GameObject in your scene, the MonoBridge detects it and makes sure all the synchronization can be done via the CoherenceSync component.