Python is easy on NixOS, you just need to use python venvs and you can use pip like normal
(python -m venv .venv) to create the venv (only need to do once per project)
.venv/bin/activate to enable the venv (Vscode should do this automatically if you create the venv through the python extension)
Then just pip install to your heart’s content
(Probably a good idea to pip freeze > requirements.txt every time you install a new library too to make it reproducible
Also you should probably add the venv directory to gitignore if you’re using git as it’ll add a lot of crap to source control that can be easily regenerated from the requirements.txt