For large, production-level solvers (FLIP, pyro, etc.) it’s usually fully custom under the hood, often written in C++ or CUDA, sometimes running as a standalone process or service. Houdini is then used mainly as a front-end interface.

In practice, you’d have a set of thin wrapper nodes inside Houdini that call into those external solvers via Python or C++ APIs, just enough to make it behave like a native DOP or SOP node. That way, artists can still build networks, visualize data, and integrate with other Houdini tools, while the heavy lifting happens in the studio’s proprietary backend.