How does a ball bearing door hinge work?
The casing is very thin and its purpose it to keep the bearing's balls in place, protect them from dust and grime and to hold in some lubricant. It doesn't share any load. The hinges are very durable as the friction load is much less than with a regular hinge. They are also quieter.
Below are a few diagrams on what might be inside a ball bearing hinge:


Hope this helps.