Either use Anaconda (if you’re using a lot of science libraries, or you also want to install non-python stuff like git, pandoc, or nodejs), or pyenv, which will install and manage multiple versions of Python for you in parallel. In fact pyenv can also install Anaconda, so that’s another possibility. If you’re just running Jupyter notebooks for doing some science or data analysis, I’d go with Anaconda. If you do a lot of library development and testing, pyenv is probably better.

Don’t use the system Python, brew-installed Python, it the official Python installer, if you can avoid it (you’ll want your Python to be up to date and self-contained in your home folder as much as possible)