Where can I see modifications made to the PATH environment variable by the Go installer
The /etc/profile file, which Bash executes for login shells, includes the following content:

# System-wide .profile for sh(1)

if [ -x /usr/libexec/path_helper ]; then
    eval `/usr/libexec/path_helper -s`
fi

if [ "${BASH-no}" != "no" ]; then
    [ -r /etc/bashrc ] &amp;&amp; . /etc/bashrc
fi


It's the path_helper program that sets up the PATH environment variable.

man path_helper:


  The path_helper utility reads the contents of the files in the
  directories /etc/paths.d and /etc/manpaths.d and appends their
  contents to the PATH and MANPATH environment variables respectively. 
  ...


In particular, the Go installer creates a file called go in /etc/paths.d with the following content:

cat /etc/paths.d/go 
/usr/local/go/bin