What tools and workflows can help me efficiently design and manage large open-world levels in Unity?
And
How do I create seamless transitions between different areas or scenes in my Unity game world?
Check out this simple coding example on the Asset Store:
For large open worlds, terrain streaming/scene streaming will be the biggest thing as far as workflow goes. Now tools will be your choice, as theres tons out there. As kdgalla posted one, there are more like SECTR, World Streamer. etc. Just use tools you like and can use effectively. If you use search here on the forums, have a look around, there have been many many posts about this subject.