How do I get the definition of a function ?

[gnuist006]

How do I get the definition, ie source code of a function in emacs
lisp ?

But if it shows some binary or opcode string.

How do I decompile the string ?

Please paste the session of an example
using eval last sexp or eval print last sexp.

Also what is the mathematics of the opcodes ?
Is it related to the combinators in any way ?
Is there any mathematics to speed up the interpretation
of these opcodes and keep their size minimal? Is there
any relation to the mathematics behind the lisp machines?

Just want to see the whole picture.

Thanks a lot for elucidation.

Gnuist