Generally, the virtualenv package is not required when using python3.3 or later, since it was incorporated into the standard library via PEP 405. As you note in the question, there are some relatively small differences between the latest versions of virtualenv and the venv package in the standard library. In part (e.g. --no-site-packages) this stems from the different implementations. Since venv is in the standard library, it doesn’t have to jump through some of the contorted hoops that virtualenv does in order to create a self-contained python installation, such as copying much of python’s site module.
The best resource is really to read the PEP thoroughly.