Where can I find the code for an animation like this?

I am not sure, but this looks like something that could be created with blender. Is your query how you would recreate something like that animation? or is it to see the underlying code?

https://www.blender.org/features/simulation/

You could probably create something similar and then find the source code files if that is what you really want to see.

If the goal is to write code, I'd suggest considering the Processing language. https://processing.org/

Check out particles.js and or p5.js both might be good starting points