First they install a solid layer of 1.5" polyiso across the roof and tape the seams. I don't have a photo of this step.
Then they install nailers which the second layer of 1.5" polyiso in place and tape those seams as well. The function of the nailers is to give the roofers something to easily attach the roofing to.
This method also allows us to add an overhang with no thermal bridging, no direct connection from the inside of the house to the outside of the house. This way the rafters cannot conduct heat to the inside or outside whichever is colder.
It is a fairly complicated working on the roof the this way, but it seriously enhances the thermal boundary of the house. We do have porches on 3 sides of the house which makes this work so much easier.
In the photo below you can see the underside of the rafters and how the overhang is built.