I’m currently playing around with articulation bodies which were introduced in 2020.1. Some parts of the feature are still buggy or not completely implemented yet. Especially the spherical joint seems to exhibit a lot of bugs that crash Unity’s physics system and should be avoided at the moment.
Because the feature is so new there is very little documentation. I hope to get you a headstart by collecting the best information sources I found so far. If you want to evaluate if the feature is suited for your needs before reading the documentation, take a look at my articulation body overview for a short summary of the advantages and constraints.
The most helpful source for me was the Unity announcement thread about the new articulation system. It’s maintained by a Unity dev that helps to develop the feature. It’s long but I encourage you to read the whole thread. It’s the best source on the topic and the best place to seek help and request features. There is also a Unity blog post with a more general explanation of the new feature set.
If you want to do a deep dive into the underlying functionality, the documentation of Nvidia is also great. Nvidia PhysX 4.0 is the articulation body system Unity is integrating.
Unity’s documentation on the scripting API is ok but the manual is better. Be aware that a few of the documented properties are not yet implemented. For example the inverse dynamics and the joint forces.