A pex is a single standalone file that contains both your code and resource files and any 3rdparty dependencies that your code needs. Normally you build a pex from a

python_binary

(which provides the entry point - the "main" module). The pex will contain the code from all the

python_libraries

that binary transitively depends on, and also any 3rdparty dependencies that that code transitively depends on. So a .pex file is completely self-contained and you can deploy it simply by copying it to a machine that has a python interpreter on it. No further downloading needs to happen - it all happened when the pex was built.