You are comparing apples and oranges, obviously. Perhaps some people are just unconsciously too enamored with apples.
OP's problem was not configuring default Python version as in your second link. Your first link is about some IBM stuff, and IBM is proprietary, so no comments there.
Neither was OP's problem about setting up Python on an old version of the Mac OS.
Configuring the default python, also referred to as installing a new Python, is exactly what OP was doing.
The process in the supplied link, which is the same as the process you weirdly called IBM proprietary even though it has nothing to do with IBM, is essentially the same as the process on MacOS. Realize that the OS python is not what you want, enable a non-default package repository with a few shell commands, install Python.
Prejudice about anything to do with Apple aside, glad to see we agree that RHEL and MacOS have similar install processes.
OP's problem was not configuring default Python version as in your second link. Your first link is about some IBM stuff, and IBM is proprietary, so no comments there.
Neither was OP's problem about setting up Python on an old version of the Mac OS.
In any case, on old RHEL, you can just do something like this: https://www.2daygeek.com/install-python-3-on-centos-6/
Installing Python on Linux is much easier than you're trying to say.
For the the love of apples.