Installing viritual Python3 environments of different versions

guide_venv_versionDiff.txt

Source: https://www.roelpeters.be/virtualenv-venv-choose-python-version/
Documentation: https://virtualenv.pypa.io/en/stable/

python -m pip install --user virtualenv
python -m virtualenv --help

py -m virtualenv -p=<your_python_executable> <virtual_environment_directory>

Example use:
py -m virtualenv -p="C:\Program Files\Python37\python.Exe" .virtenv
.virtenv\Scripts\activate

Or more simply:
py -m virtualenv .virtenv
.virtenv\Scripts\activate

Run to save inventory of modules in use:
pip freeze > requirements.txt

Run to install from requirements.txt tile in fresh installs.
pip install -r /path/to/requirements.txt

Ubuntu:
python3.7 -m virtualenv virtenv
virtenv/bin/activate

This is also useful to consider when using the viritual environment within a Git:
https://stackoverflow.com/questions/51863155/do-we-need-to-upload-virtual-env-on-github-too