You are not getting any files against those commands because they are shell (bash) built-ins, not separate executable files (e.g. binary files, scripts). Actually, shell built-ins are compiled into the shell executable; if you want you can check the source code to be sure of it. As which or whereis only looks for external executable files, you are not getting any output for the built-ins.

To find if a command is a built-in or alias or function or external file, the best way is to use the type built-in:

$ type fg

fg is a shell builtin



$ type bg

bg is a shell builtin



$ type jobs

jobs is a shell builtin

Also note that your find command is not syntactically correct. The correct (simplest) syntax is find /where/to/search -name 'name_to_search'.

Also note that few commands are implemented as both a shell built-in and a separate standalone executable. For such commands, always remember that the built-in command will take precedence over the external one. So, when you run echo something, the built-in echo is run. If you want to run the binary executable echo you need to call it in a different way. One way is to use the full path to the executable: /bin/echo something.

To display all available versions of a command, run type with the -a option:

$ type -a echo

echo is a shell builtin

echo is /bin/echo

To get documentation for shell built-ins, you can check the manpage of bash or use the help command (which is a built-in command):

help jobs

Also as @terdon pointed out you should use type instead of which.