> Furthermore I do not understand why the simlink is created. I suppose that `python3` is already on the `PATH`, so what's the purpose of simlink?

On machines with multiple Python installs (eg Python 3.6 and Python 3.7, or a distributed Python 3.7 and a user-built Python 3.7), `python3` from PATH may refer to the incorrect installed version. Having a symlink to the the Python executable which built the environment (by default; `--copies` overcomes this) forces an executable.

This `python3` symlink still makes it distributable to other machines on the same platform anyway: `/usr/bin/python3` should always be available on other distributions with Python installed normally, and Windows copies the Python executable anyway AFAIK.

Across-platform is likely not going to work anyway: it's likely that your virtual environment will contain platform-specific installations of the packages, meaning they won't work on other platforms. If you just want Linux/MacOS portability, check out the Pex project.

I agree with VIRTUAL_ENV being relative however