tinyraycaster VS UnityCsReference

That sounds like a fun challenge. If you're constraining yourself to use as few libraries as possible, I'd go with OBJ [1] for the 3d mesh and PPM [2] for writing images. It's easy to implement a bare bones reader/writer and some OSes (like macOS) can show them in the file browser. Raytracing in One Weekend goes over PPM. There are a bunch of header-only libraries that handle different file formats like stb_image [3]. I usually end up using those when I start dealing with textures or UI elements. I don't use Windows so I haven't used their APIs for projects like this. I'd usually go for imgui or SDL (like you mentioned). tinyracaster, a sibling project of tinyrenderer, touches on those [4]. I liked LazyFoo's SDL tutorial [5]. Good luck!

[1] https://en.wikipedia.org/wiki/Wavefront_.obj_file

[2] https://en.wikipedia.org/wiki/Netpbm#PPM_example

[3] https://github.com/nothings/stb

[4] https://github.com/ssloy/tinyraycaster/wiki/Part-4:-SDL

[5] https://lazyfoo.net/tutorials/SDL/index.php

I haven’t followed this one but I did follow another of his tutorials and that was good, here is ssloy’s tinyraycaster: https://github.com/ssloy/tinyraycaster/wiki

Don't just jump into the source file , read the make file first, from there you'll know how main file is dependent on other files , and start with the .h(header files as they'll give a basic idea what their corresponding .cpp file do and yeah start with small Repos first as they are easier to understand like this one.

I believe this wiki page can walk you through a retro style raycaster engine: https://github.com/ssloy/tinyraycaster/wiki

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.