The definitions are scattered through the Arduino installation, but for example the register addresses you asked about used are defined under board-specific files under hardware\tools\avr\avr\include\avr\.
They aren't documented as part of the Arduino runtime environment, I suppose on the basis that they aren't part of the API - they are part of the implementation. It's possible to bypass the Arduino library and access the hardware directly, and obviously various clever people have done that - and some of them have documented what they did and how it works - but those definitions are not documented as part of the API.
I see. So, if I read you correctly, the example I read on setting the timer to execute an interrupt is bypassing a normal way to do it. Obviously so did I
. So I should look for a library the makes it easier to set up an interrupt routine then?
You mentioned the __DATE__ and __TIME__ preprocessor directives. There are some macros and directives which are defined by the C++ language, and others that are added by the GCC compiler that Arduino uses. These are also not part of the Arduino API and so are not covered by the Arduino documentation, but you're free to look up the language specification and compiler behaviour for yourself if it interests you.
Ah, so I will look up some references on C++ and GCC then. I guess it never occurred to me, being new to how it all works, that I could use native C++ stuff inside a sketch.