You do not need manual version management if you have multiple versions of Python installed. By default, it will use whichever interpreter supported by the application is listed first in PYTHON_TARGETS. If you want, you can override that by calling the python interpreter you want manually, eg `python3.8 <program name>` or `pypy <program name>`. But if python3.9 is the "default" interpreter (because it's listed first) but the application only claims support for python3.8 in the ebuild, if you just run the application with no qualifiers it will start the python3.8 interpreter.

Obviously there were many, many years when python2 and python3 needed to be installed side by side, and it's reasonably common for people to have multiple versions of python3 installed side by side. My VPS has both python3.9 and python3.8 installed side by side, for instance, because apparmor is slow to pick up python3.9 support.