"Note, there are two versions of manim. This repository began as a personal project by the author of 3Blue1Brown for the purpose of animating those videos, with video-specific code available here. In 2020 a group of developers forked it into what is now the community edition, with a goal of being more stable, better tested, quicker to respond to community contributions, and all around friendlier to get started with. See this page for more details."
specifically it's a community variant of it. Grant still has his own version (https://github.com/3b1b/manim) where he works independently, and the community fork builds off that and e.g. adds and maintains things Grant might not care about.
Edit: sorry for being a joke but I saw "Python library math animation" and I immediately thought of 3B1B but I really had no idea this was it!