which python vs PYTHONPATH

You're mixing 2 environment variables:

• PATH where which looks up for executables when they're accessed by name only. This variable is a list (colon/semi-colon separated depending on the platform) of directories containing executables. Not python specific. which python just looks in this variable and prints the full path
• PYTHONPATH is python-specific list of directories (colon/semi-colon separated like PATH) where python looks for packages that aren't installed directly in the python distribution. The name & format is very close to system/shell PATH variable on purpose, but it's not used by the operating system at all, just by python.