I'm certain the host heavy lifting is done by numpy which is a python wrapper around Fortran and C. The visualization heavy lifting is done by pygfx/wgpu-py. wgpu-py has C. I think wgpu-py compiles to WASM to run in the browser. More and more packages are taking this route.
All true, except the bit that wgpu-py compiles to WASM. It's all desktop.
In the plans that we do have for running the browser, Fastplotlib, Pygfx and wgpy-py will still be Python, running on CPython that is compiled to WASM (via Pyodide). But instead of wgpu-py cffi-ing into a C library, it would make JS calls to the WebGPU API.
[1] https://github.com/pygfx/pygfx [2] https://github.com/pygfx/wgpu-py