1966 Dart D-Dart cam spec

Not to steal a good thread here but I have a question about grinding cams.

If you send in a core with smaller lift heights and smaller base circles- how does a machine shop “regrind” a cam larger?
For $79 they are not welding to your cam AND regrinding it. Anyone care to explain?
The cut down the base circle even more. (There is a limit of course.) If you take a peak at large roller cams, there base circle is nearly down to the maximum, practically sitting the lifter on the uncut portion of the cam.