Whether PIP is part of the embedded version of Python is unknown to me.
Unfortunately there is no pip at least on macos and Krita do not “see” installed pip/packes out of the box. And i failed to install it via get-pip.py or similar tricks that worked for blender for example (although recent versions have pip from the start). sys.path expansion works both on MacOs and on Windows
Python 3.9? Then everyone with an up-to-date Python has to hold a Python 3.9 in a different folder
Yep, Krita 5.1.5 uses 3.9 (for windows this is valid for 5.2.0. 5.1.5 on windows still uses Python 3.8) and packages added with sys.path expansion should match it. But there is conda/virtual envs to keep different versions at the same time, so this is somewhat managable, both on mac and win