Notice how these are just HTTP links to files on a CDN. This reduces the headache of downloading files, moving them around, adding the path to your project, etc. You just drop the link in, and you're good to go (assuming you have internet connection while you are developing).
In the big purple book Node.js, MongoDB, and AngulasJS Web Development, Brad Dayley describes something called a bootstrap phase in the life cycle of an angular application. Each time the web page is loaded into the browser, three phases occur- bootstrap phase, compilation phase, and runtime phase. The bootstrap phase is basically the browser loading up the script tag to the Angular library files, initializing basic under-the-hood Angular components, and initializing your module that you specified with the ng-app directive.
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