The physics is MuJoCo, compiled to WebAssembly by DeepMind and running in this tab. Your model and your code never leave the browser.

Your controller is plain JavaScript. It runs once per control tick and returns what each motor should aim at — the same shape as a real policy, without a runtime to install.

Simulation uses the collision geometry, not the visual meshes. That is what a physics engine acts on, and it is where most models turn out to be wrong.