uGUI VS UnityCsReference

I also looked for if there is the documentation on the github repo, but it seems they have hosted the docs somewhere else (which is maybe not open-source).

Here's the source: https://github.com/Unity-Technologies/uGUI/blob/5ab4c0fee7cd5b3267672d877ec4051da525913c/UnityEngine.UI/UI/Core/InputField.cs

The UI sources might be the best actual existing documentation on what's available, take a look here for example: https://github.com/Unity-Technologies/uGUI/blob/2019.1/UnityEngine.UI/EventSystem/EventInterfaces.cs

I've checked their CS reference code on their Github, both here and here, but I want to dive a little deeper - specifically, I want to know what they're actually doing when determining validity of the Matrix4x4.

Yep, but beyond just decompiling it, the c# layer of code is on GitHub: https://github.com/Unity-Technologies/UnityCsReference

https://github.com/Unity-Technologies/UnityCsReference/blob/master/Runtime/Export/Scripting/UnityEngineObject.bindings.cs (after 2019.1)

The code where the method filtering for the Animation Event Inspector is done, is here (line 190). Is it possible to do something like this? That repository explicitly says:

No reason, you can look at the source of encapsulate

There isn't really a straight forward way to do this, even the QualitySettings editor uses Serialised Properties to get at the data. (Line 343 in QualitySettingsEditor)

This is awesome, thank you! I'm most curious about their Runtime

I think if you want to get into graphics programming you do want to work with OpenGL and similar things, because at the very least you need to understand it all (and decide what parts of engines to use and what to ignore when you get to whole games). It's also worth saying that while you can only publicly get the references in Unity you do get source access at the higher subscription tiers you'd use at a game studio.