Nested diaphragms are used to produce convolutions. These are welded into the bellows.

The diaphragms are often described as “crinkly washers” but are in fact precision pressed components having inside and outside diameters with a rippled profile formed in the surface.
Usually very thin materials are used in the production of the diaphragms, typically 0.05 mm to 0.25 mm thick in Stainless Steel, but often in more exotic alloys like Monel, Hastelloy, Inconel, Titanium etc.
It is usual to have a so-called Male and Female  form to the diaphragms ensuring the bellows fully nests without the diaphragms interfering.