This happens syntactically, for classes known to the compiler.

instance definitions are added for every data constructor produced
by "data". 

https://www.haskell.org/onlinereport/derived.html#derived-appendix

http://stackoverflow.com/questions/3864647/how-does-deriving-work-in-haskell

In GHC, https://wiki.haskell.org/Template_Haskell  can produce the necessary syntax:
https://wiki.haskell.org/Template_haskell/Instance_deriving_example

For Prelude's types like integers, tuples, lists, strings, booleans, etc., 
https://hackage.haskell.org/package/base-4.6.0.1/docs/src/GHC-Show.html
shows the code.