Can we see the definitions of predefined functions in C language?

Answer 1

If the compiler is open source, yes you can get it and modify it to your hearts content. This is where GNU Compiler Collection is maintained.
https://github.com/gcc-mirror/gcc

Answer 2

strlen is here
https://github.com/lattera/glibc/blob/master/string/strlen.c

Answer 3

No we cannot see how a predifined function is written in C. But we can recreate it just as I have recreated the strlen() function.