Where can I find the actual bangle.js file/code?

If you want to find out where something specific is defined, to find out how it works, to play with it to improve the documentation, you can just find it at https://www.espruino.com/Reference#software then in the title you'll see an icon like ⇒ - click that and it will take you to the exact place in the code where the function is defined.

But in many cases (especially for E. and Bangle.) the code is probably not JS, but C