I know how to use categories as a programmer, but I'm curious how they are implemented. Does the compiler compile them into calls to class_replaceMethod from a static initializer? Thanks.
New answer on topic.
Each class has a list of methods, when doing a method lookup the method list is scanned from beginning to end. If no method is found the superclass' list is scanned, etc. until reaching the root class. Found methods are cached for faster lookup next time.
When loading a category onto a class the categories method list is prepended to the existing list, and caches are flushed. Since the list is searched sequentially this means that the categories method will be found before the original method on next search.
This setup of categories is done lazily, from static data, when the class is first accessed. And can be re-done if loading a bundle with executable code.
In short it is a bit more low level than class_replaceMethod().