Open this: http://nesdev.parodius.com/mappers.zip It contains a file that has extensive documentation on the registers and functions of many, many different mappers.
Apparently the 6502 itself plays no role in memory mapping except to write values to the mapper registers (which are located in the ROM space ($8000-$FFFF)); I presume once the value(s) has/have been written, the mapper and the 6502 go on their own merry ways.

I think the mappers' primary role is to control what the CPU has access to; think of them as a switch at a fork in a railroad track. The word "gate" also comes to mind. I can't think of a better way to explain it.
*shrugs* I hope that document helps you out.