How would I see the code behind these functions?
You would need to find the source code for the relevant project (Firefox, Chromium, V8, SpiderMonkey, etc.), provided the project is open-source.
Would I need to understand the language that an engine such as V8 is written in and read through that, or is there a simpler solution.
Yes, you would. Note that alert is not a JavaScript function, it's a function defined by web browsers (so you'd need to look at the Firefox or Chromium, etc., source). Object and String are both part of JavaScript, so you'd look at the JavaScript engine sources for those (SpiderMonkey [or whateverMonkey, the prefix seems to change a lot] and V8, etc.).
