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How to create a Python virtual environment independent of OS and Python version

I am trying to create a virtual environment to run a script which requires Python 3.6. I started off with Pipenv but I am unable to create the same environment on other platforms via the Pipfile.lock or requirements.txt unless the other platform(s) has Python 3.6 installed. I have read this post but I am unsure which direction I should take to create a virtual environment which can be shared and run its own version of Python independent of operating system and version of Python installed on the other platform.

Virtual environments are not portable, they depend on the Python installation you have.
You can't share/distribute virtual environment with others, because you can't control which version of Python others are using.
If you want to distribute your code along with all dependencies including the specific version of Python interpreter, you can use PyInstaller. It is far from perfect and little bit hacky. Also it generates a package which is specific to operating system.
https://pyinstaller.readthedocs.io/en/stable/operating-mode.html
There is also a detailed step-by-step guide on how to use PyInstaller.
https://realpython.com/pyinstaller-python/

This is step-by-step how I use Python virtual environment and share it with co-workers.
To check python and virtualenv presence, run following commands:
which python3
python3 -m pip list | grep env
which virtualenv
Install a python virtual environment builder:
python3 -m pip install virtualenv
Create a virtual environment named venv inside the project's directory: virtualenv venv
To activate this environment use this command inside project's directory: source venv/bin/activate
Install python modules dependencies listed in a requirements.txt:
python3 -m pip install -r requirements.txt
You should activate virtual environment when you working with python in this directory for package installation and for running commands in the project directory. When you need to deactivate the virtual environment do it using deactivate command.
To deactivate environment simply run: deactivate

How do I activate python virtual environment from a different repo?

So am working in a group project, we are using python and of the code is on GitHub. My question is how do I activate the virtual environment? Do I make one on my own using the "python virtual -m venv env" or the one that's on the repo, if there is such a thing. Thanks

virtual env is used to make your original env clean. you can pip install virtualenv and then create a virtual env like virtualenv /path/to/folder then use source /path/to/folder/bin/activate to activate the env. then you can do pip install -r requirements.txt to install dependencies into the env. then everything will be installed into /path/to/folder/lib
alteratively, you can use /path/to/folder/bin/pip install or /path/to/folder/bin/python without activating the env.

Yes, you'll want to create your own with something like: python -m venv venv. The final argument specifies where your environment will live; you could put it anywhere you like. I often have a venv folder in Python projects, and just .gitignore it.
After you have the environment, you can activate it. On Linux: source venv/bin/activate. Once activated, any packages you install will go into it; you can run pip install -r requirements.txt for instance.

install packages in Python2 with Python3

I am trying to install numpy, nltk, etc packages for Python 2 to run a code. But I have Python3 as well and the path variable is set to it. When I try to use any pip install command it shows the package is available in Python3's directory.
Also, I am using VSCode, so I did not add the path variable.

I suggest you use virtual environments. Because if you read about virtual environments, you will find that they are created for such cases.
To create virtual environments, you must do the following:
Make a note of the full file path to the custom version of Python you just installed.
virtualenv -p /home/username/opt/python-2.7.15/bin/python venv
In order to use this environment’s packages/resources in isolation, you need to “activate” it. To do this, just run the following:
source venv/bin/activate (Linux)
./venv/Scripts/activate.bat (Windows)
Notice how your prompt is now prefixed with the name of your environment (venv, in our case). This is the indicator that venv is currently active, which means the python executable will only use this environment’s packages and settings.
Now run the following:
(venv) $ which python
/Users/ashkan/python-virtual-environments/venv/bin/python (in my case)
now you have access to python2.7.

The best practice for this particular problem would be virtual environments.And for that matter Pipenv would be a good option.
Install Pipenv.
$ brew install pipenv (MacOs)
$ sudo apt install pipenv (Debian)
$ sudo dnf install pipenv (Fedora)
pip install pipenv (Windows)
Creating virtual env with Pipenv.
pipenv install --python 2.7 numpy
This command will install create a virtual environment and install python 2.7(which will be used as the main interpreter once you activate the environment) along with numpy in that environment. This will avoid the packages version conflicts too.
To activate the environment
pipenv shell
If you are working in the Vs Code workspace then you should set the interpreter path(python path) to the path of the virtual environment.

when we install anything using pip. it will install dependencies for default python version. so you can change the default python version using this link https://linuxconfig.org/how-to-change-from-default-to-alternative-python-version-on-debian-linux
Hope this will solve your problem

After crating a virtual environment with python 2.7 you can install your required packages

python3 -m venv: how to specify Python point release/version?

To create a virtual environment using virtualenv you can specify the Python release and point version like so:
virtualenv --python=python3.6 .venv
How can I achieve this using Python3's venv module (as in python3 -m venv .newvenv)? According to the documentation using venv is the recommended way to create virtual environments but I didn't see how I can choose a virtual environement with a specific Python version.

Run venv with whatever Python installation you want to use for the new virtual environment. For example, if you would run your Python 3.6 installation with python3.6, then
python3.6 -m venv whatever
would be how you create a Python 3.6 virtual environment.

I thought to add to this answer when one is using pyenv. In my workflow I use pyenv to have multiple python versions but not to manage virtualenvs. I rather have my python virtual environment in the project's root. With pyenv one can install multiple python versions by running pyenv install 3.8.10 and after that pyenv install 3.9.0. When you run pyenv versions you should get something similar to this
system
* 3.8.10 (set by /Users/<user>/.pyenv/version)
3.8.10/envs/python-test.venv
3.9.0
When working on a project and choosing what python version should be used in that project you can do the following.
$ mkdir my_project && cd my_project
$ pyenv global <version>
$ python --version // should be the version you set as global
$ python -m venv .venv
$ source .venv/bin/activate

I was able to avoid error mentioned in the comments by using the option --without-pip. Then after activating the venv, I installed pip manually with the get-pip.py script.

How to use a Python virtual env in my Ubuntu bash?

I am trying to package my project with a virtual env so that it is easier to implement.
I am trying to do this in a Ubuntu bash.
I have succesfully created a Python venv using the Python virtualenv library.
I do manage to activate it using source venv_name/bin/activate.
I can indeed see (venv_name) at the beginning of my command line.
However, I do not manage to actually use this virtual environment.
I have for proof that when I type which python3 I get my root python3; and I have tried to update a package in the virtualenv but it has been updated in the root python.
What should I do to actually use my virtual env ? For now I am trying:
python3 myscript.py
And it is working but I suspect it's running with my root python3.

I think you have two versions of python (2 and 3). You create virtualenv with python 2. Recreate virtualenv with correct python version

You have to make next steps to use python3 virtual environment in Ubutnu:
1. Install virtual environment lib for python3 with command:
pip3 install virtualenv
2. Create your virtual environment:
python3 -m venv venv
3. Activate it:
source venv/bin/activate
Works correct for me in Ubutnu 16.04