bgfx VS Bullet

Sort of, I'd recommend a modern higher level API. I'm not sure what the current recommended ones are (probably bgfx), but assuming the wrapper is "low level enough", then the concepts you learn are still going to apply.

https://github.com/bkaradzic/bgfx for just that FOSS intermediate rendering library (includes Minecraft)

BGFX: Pretty mature and easy to use with many backends.

bgfx. I have not used it, but I have heard good things about it.

There's kind of a lack of this for C++ in 3D, I think it's often due to the necessity of a secondary scripting language in game engines with C++, which isn't necessarily needed in Java or C#. SFML is like that (but also 2D), Godot is similar (but more geared towards 2D). Ogre3D is an actual engine like I mentioned earlier, not sure how easy it is to use. Cocos2d is higher level, but is also 2D only. I'm not fond of SDL, it feels like a windowing library with slow old school immediate mode stuff attached, so it ends up not being good at the rest of the tacked on things. SDL is popular as a windowing library, and it's why you see it used everywhere (but the most notable uses of it aren't using their drawing capabilities), I often see bgfx thrown around, and for you it might be a good choice, though I have no experience with it.

Another route could be using an abstraction over Vulkan (faster, more efficient, more difficult): bgfx, dawn, magma, or wgpu (Rust).

now, java is actually quite a performant language and even if its not most of the performance bugs in mc are due to it being single threaded, inefficient chunk generation and optimizing, and it built ontop of opengl WHICH isn't much of a performance hit but its still ehh idk it doesn't matter that much (NOW SWITCHING THE GAME TO AN ENTIRELY DIFFERENT GRAPHICS API WOULD SUCK ASS TO DO (and vulkan is quite verbose :))) (AND also bgfx would probably be better due to it being an abstraction layer ontop of all the graphics apis so minecraft could target many depending on your platform (and also bedrock used to (or still does i dont know) use bgfx before they switched to just two (IF IM READING MC WIKI RIGHT BECAUSE IM NOT ENTIRELY SURE IF THEY USE BGFX STILL ?? SO THEY COULD STILL BE TARGETING MULTIPLE YET THEY JUST WROTE THEIR NEW SHIT BAD IDK))

For typical game physics engines... not that much. Math libraries like Eigen or Blaze use lots of template metaprogramming techniques under the hood that can help when you're doing large batched matrix multiplications (since it can remove temporary allocations at compile-time and can also fuse operations efficiently, as well as applying various SIMD optimizations), but it doesn't really help when you need lots of small operations (with mat3 / mat4 / vec3 / quat / etc.). Typical game physics engines tend to use iterative algorithms for their solvers (Gauss-Seidel, PBD, etc...) instead of batched "matrix"-oriented ones, so you'll get less benefits out of Eigen / Blaze compared to what you typically see in deep learning / scientific computing workloads.

The codebases I've seen in many game physics engines seem to all roll their own math libraries for these stuff, or even just use SIMD (SSE / AVX) intrinsics directly. Examples: PhysX (https://github.com/NVIDIA-Omniverse/PhysX), Box2D (https://github.com/erincatto/box2d), Bullet (https://github.com/bulletphysics/bullet3)...

I suspect most C++ physics libraries like Box2D (https://github.com/erincatto/box2d) or Bullet3 (https://github.com/bulletphysics/bullet3) could really benefit a lot from SIMD.

No, they replaced Bullet with Jolt. That is considerably more than "some adjustment", regardless of what you think of the result.

Vulkan graphics via Intel GVK, and physics via Bullet

The first two have a bunch of great examples, and I’m tying them together by refactoring some of the THREE examples to fit the ECS paradigm defined in AFrame. then upping the ante by adding physics using AMMO, which is more challenging since it’s only a partial implementation of Bullet, and already poorly documented (yet popular) physics engine.

Plus, SM uses a system called bullet physics, I imagine this would be rather complex to rework into a modern engine such as Unreal or Unity (after all, the majority of performance issues come from the physics engine rather than the graphics engine)

The physics in HDT-SMP are actually being calculated outside of Skyrim's engine with Bullet, an open-source physics engine. So this isn't some limitation of Skyrim's engine.