They are soldered and, probably, laser welded together.
Five bands are prepared, the four top ones and the thin underlying
band. They are then assembled. The diamonds may be set either before
or after assembly.
The band of what looks like calibre cut stones, which they call
"Clou de Paris" (Parisian nail), is actually a band of gold surfaced
by physical vapor deposition (PVD). This is the most technically
difficult part of the process as it requires specialized industrial
If the deposition material is sufficiently stable under the heat of
soldering then that band could be coated before assembly. If the PVD
coating would be compromised by the heat of soldering then it may be
possible to assemble first and then mask the rest of the ring during
coating, similar to masking off part of a piece while
My suspicion is that both soldering and laser welding are used
during assembly to keep the heat away from the PVD surface.
Because of the PVD coating, the ring's construction is highly
technical but not really mysterious.
Wikipedia has a decent article on PVD coating.