Is it a matter of using thick sheet and filing the edges to give
the appearance of a domed outer shape?
Yup. Pretty much. Start with thicker bar stock (which amounts to
thick sheet, I suppose), and file inside edges of the band to get the
comfort fit. The outside is formed or filed however you like. If you
form the band first, you can then use either half round files, or
rotary files or sanding drums to get the inside curve.
Or are there forming tools out there for such a process?
Specialty rolls for rolling mills can give you a half round wire
shape (or lower dome than half round) If you make a ring of this,
with the curved surface on the inside of the shank, you end up with a
comfort fit ring with a flat outside.
You can also use oval drawplates to make an oval wire. Bent into a
ring, both inside and ouside are then domed, like a standard comfort
fit plain wedding band. I’ve even see drawplates made especially for
this use, where it’s not strictly an oval, but a suitable curve on
the inside for the comfort fit, and usually a deeper curve for the
outside domed surface of the ring.