Where modules reside

Within every Python installation is a folder called Lib, and within that folder is a folder called site-packages. On my machine, the folder sits at C:\Python27\ArcGIS10.5\Lib\site-packages.

Almost all third-party modules are copied into this folder to be imported as needed. The main exception to this rule, for our purposes, is the ArcPy module, which is stored within the ArcGIS folder in the Program Files folder (for example, C:\Program Files (x86)\ArcGIS\Desktop10.5\arcpy). To make that possible, the ArcGIS installer adjusts the Python system path (using the sys module) to make the arcpy module importable, as described next.