I don't use Conda, but to answer your first question: The recommended Python version for any project should no longer be Python 3.9, as it will reach end-of-life status in October 2025.

Of course, there may be project or use case specific reasons to use a different version. For example, there might be a required package that does not support later Python versions. In that case, I would try to fix/replace that package, rather than lock the Python version into something that will reach end-of-life in a few months.

As I said: You need good reasons to use an "old" Python version. If you don't know those reasons for your project, start with the current version (which is 3.13 as of today).