Thanks for your 10.75 megabyte link that came complete with pics demonstrating the process. I gather the following:
Builders use wooden forms to form a temporary vaulted ceiling. Then, after they lay a layer of cement on the forms, they laid on the ‘vaulting tubes” which were flexible and provided some strength. Another layer of cement goes on top of the tubes. Remove the wooden forms on which the various layers rested and and you have a smooth vaulted surface of cement on top and bottom of the tubes.
God, I love the internet.
I agree fascinating world we live in.