The magic is in all the code that interprets those annotations. So you have to understand a large and complex framework to understand which objects are being injected (in Spring, for example, you have to keep in mind the rules about which beans are instantiated and then which of the instantiated beans are chosen for injection into a particular class), and to understand what actually gets injected (is it actually the object returned from the annotated method, or some proxy or chain of proxies adding additional behaviour to those objects?).

The principle of dependency injection is a good one, but dependency injection frameworks are an unnecessary and overcomplicated way of achieving it. XML-based DI frameworks at least had a rationale - the idea was to externalise configuration, so you could change the dependencies without changing the code. It turns out this kind of externalisation isn't actually valuable in most cases (changing the dependencies is just as complicated and risky as changing code, and often needs to be coordinated with more substantive code changes, so the separation is artificial).

Once you determine that you should wire up your dependencies in code, though, there's no need for dependency injection frameworks - you can just directly write the code that wires up the dependencies.