`virtualenv` still exists and is still actively developed. It's true that Python 3 ships with `venv` but I think `virtualenv` offers some additional features.
I went through the same progression, thinking pipenv was the official solution before deciding it wasn’t. Then, just now, I realized that pipenv [1] is currently owned by the Python Packaging Authority (PyPA) who also owns pip [2] and virtualenv [3]. I don’t know the right answer but this illustrates the confusion of not coalescing around an official solution.
[1]: https://github.com/pypa/pipenv
I could be wrong, but virtualenv, like pip, is not part of the core language. Its developed exclusively by pypa.