Is cam regrinding purely subtractive?

[David in San Antonio]

I searched past threads but couldn’t find the answer:
I was told that when a Model A cam is reground it is purely a subtractive process. That is, the lobes are not first built up with additional metal prior to grinding. This sounds like the reground cam could have a different shape but at no point could its diameter exceed that of the original.

Q1. Is that true that regrinding an A cam is purely subtractive?
If so, is this true for other engines?

Q2. If they are not built up, isn’t there potential for the lift height to be diminished? I presume the lift height generally isn’t increased or the valve could strike the head, especially if it’s a higher compression design. But don’t we want to at least maintain the original lift?

Q3. If not built up wouldn’t this limit the duration the valve is open? I’m picturing a pointy lobe with steep ramps vs a more rounded, wider nose, holding the valve open longer.

Thanks for helping me understand.