UnityCsReference VS NancyFx

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.

Nancyfx - sadly dead in the water now but well worth a read of both the code and docs https://github.com/NancyFx/Nancy/tree/master/

2-3 years ago Nancy added a large banner at the top of https://github.com/NancyFx/Nancy -

Being open source doesn't change the support model. It allows willing contributors to step up and fork in the event of total abandonment. As a comparison, nobody forked NancyFx to extend support or keep it alive.

It sounds like you are talking about asp.net? If you don't want to bother with controllers and attributes and all that you can just use the Microsoft.AspNetCore.Http and Microsoft.AspNetCore.Routing namespaces. As someone that really liked Nancy I felt that this was close enough.