Far as the libraries used Apple's all over the place and it really depends on what part of the site or subdomain you're on. Some parts are heaving on jQuery others older JS libraries (partially due to legacy phasing out old code), and iTunes is powered off Ember.js which is a branch of SproutCore. IIRC Facebook bought one of the core teams that were one of the big supporters of SproutCore, which is interesting since Facebook is pushing React.js as well. Far as prototype and script.aculo.us, those projects have been alienated. Thomas Fuchs hasn't updated script.aculo.us 1.x in probably 5 years so I wouldn't call that common in todays framework standards.
A lot of the Apple product pages use CSS3 for their subtle animations/transitions and it's pretty interesting that they take different approaches for similar effects on different product pages. Like others have said, Apple is probably working with smaller high end shops that are specialist but that doesn't mean every shop will code exactly the same as the other shops, even if Apple is providing Style Guide. It's no surprise the 3rd party boutiques keep it on the down low if they're creating new product sections (like when the Mac Pro was first released) but I bit it's a real tight lip involved process far as deploying to Apples stage environment.
Coming back to the OP's question, considering there is probably hundreds upon hundreds (if not thousands) of designers, front end, back end and fullstack developers working in their entire web ecosystem. I'm sure the code editors have a large range from text editors to the IDE's. If Apple is heavy on the backend with Java, I'd bet JetBrains IntelliJ very popular and WebStorm for the front end developers. Then you have developers who prefer lightweight text editors like SublimeText or the new kid on the block Atom. I wouldn't be surprised if some people coding are using Dreamweaver or some going full hipster and running Vim (MacVim), no disrespect to the seasoned Vim users.
Text editors and/or IDE's tends to be
a very subjective topic and the people that use them tend to stick to what they feel the most comfortable with. I'm sure if you did a poll you'd see a ton of products, though I'm betting JetBrains IntelliJ is their top IDE and far as text editors that's really hard to say. Sublime has been the goto choice since TextMate "Allan Odgaard" seemed to take a sabbatical to decompress. That's the sense I get with the SublimeText dude (Jon something) and Atom has really taken off and has skyrocketed within the community and that's generally what dictates what text editors rule on top.
The IDE or text editor doesn't define the developer but think of it as a toolbox. Some are the swift army knife with full on
IntelliSense (that's one of the perks of using an IDE), refactoring, real time robust debugging tools (outside of linting), VCS, extensibility and everything else in between. My day to day editor is Atom (historically went from SublimeText -> TextMate -> BBEdit) and I use WebStorm quite a bit for when I'm doing heaving debugging sessions, refactoring or just true IntelliSense (think MS coined that term).
End of the day use what you fell the most comfortable with and most importantly what you're the most productive with. Heck you could get away with just using Text Edit in plain text mode.