Yes, python has been on MacOS (OS X) since day 1. Some parts of the OS rely on python scripts, and these have stayed the same for many years. For instance, the Automator actions for Combining and watermarking PDFs have python scripts at their core.
The benefit of writing for the system-installed version is that you know it will work on a wide range of Macs: I have written python scripts that work on every OS from Leopard (c. 2007) to Catalina, with no extra installations or modifications!
Python2 has now been end-of-lined, and Catalina is the last version to include it (so Apple say). Catalina comes with python3, but the OS may not come with any python version in the future.
IMO, it's easiest to download the installer package from python.org for updates to either python2 or python3.