PhysX

Havok, Bullet and several others. Unreal has it's own Chaos physics engine. PhysX was open-sourced by Nvidia[1] but of course that doesn't help games built with it earlier, they'll just fall back to software mode if played on a series 5 GPU. [1] https://github.com/NVIDIA-Omniverse/PhysX

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.). Typically 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 minimal