In general, shells have some basic configuration files in /etc on most Unix like systems. Important file name patterns include anything mentioning the shell name, and for bash at least, the file "profile". These same files might be rendered as ., such os .cshrc, .bashrc and .profile, etc.
Bash, when started as a login shell, sources
/etc/profile, if that file exists. After reading that file, it sources for
~/.profile, in that order. In those files, the PATH can be altered, but it can also source other programs to set paths. If it's an interactive shell, but not a login shell, it will source
In the case of the Mac, inspection of /etc/profile reveals a program called "path_helper", which man page reveals that it likes to store extra paths in /etc/path.d.
That pattern, .d, is also widely used, and is a good place to look for further configuration files.