One thing to be mindful of is that you can get a simulation to behave in (almost) any way you want if you set the parameters right, so you should take care to understand the assumptions that you're baking into your sim before taking its results as gospel.

> what would happy to X if it did this? Would it have friction, etc?

MuJoCo does not simulate at a scale where you can discover whether something has friction or not. Friction is a parameter to MuJoCo's simulations.

Build things yourself. Using LLMs doesnt help you understand anything, they will just give you an annoying case of dunning kruger. Using them will only make you retart-d