I've got the following on my table:
Those are listed in reverse preference order.
ExpressPCB's RoHS process uses 99% silver. It's still copper underneath, but it may make sense to use silver-bearing solder like the common 62-36-2 eutectic blend for such boards.
Tangent speaks the truth.
In industrial process, the boards have very thin copper
on them before they are etched. Saves on copper and time.
Then they are plated back up to whatever thickness is required.
They can be plated in any coating the customer requires; tin,
silver bearing tin, silver, gold, unobtainium (if you can afford it)
or whatever else they have up their sleeve.
Other than the copper layer, most of the more exotic stuff is
plated on after the solder mask is applied so only the exposed
pads are plated with silver, gold or unobtainium.