Is the version of Python used within Cinema 4D R21 "built-in" to Cinema 4D, or can it be updated to a new version?
If it can, how is it done, and is there backwards compatibility risks with exiting plugins, scripts, etc?
Thank you for reaching out to us. The Python interpreter shipped with Cinema 4D is not identical to CPython and it is not intended to be replaced by the user. Technically speaking you could try it, but this would be out of scope of support and will likely also fail. We cannot give instructions on how to do it, you would have to hack your Cinema 4D yourself.
It also depends on how much you want to change the Python version. Given that you are on R21, i.e., Python 2.7, and likely want to jump to a Python 3.X version, the answer will change from "practically impossible" to "categorically impossible".